For those of us who intend to keep their Jaguar for a long time, consider using Evans Waterless Coolant. This coolant is advertised in Jaguar World, used by Jay Leno in his classic car collection, etc.
Evans is used 100% with no water dilution. Its boiling point is well in excess of 250 degrees. It does not materially degrade over time. Evans claims it last 250,000 miles. It is not cheap, costing about $35/gallon.
A principle advantage with Evans is the high boiling point. Conventional coolants require a presurized coolant system to raise the boiling point. Coolant under high pressure will find leak points. Evans does not require pressure - you leave the coolant reservoir cap loose; and, therefore, Evans is less prone to leakage. This has some importance in water pump/thermostat life.
A lot has been written about "hot spots" in cylinder heads where coolant circulation is slow allowing temperature to exceed boiling points, causing steam which creates a pocket empty of coolant. A product called "Water Wetter" seems to help - I think it is some sort of detergent which breaks down the surface tension of water. With its much higher boiling point, Evans is less prone to create these hot spots.
I now have 8,000 miles or so with Evans in my '07 XK. I have a ScanGauge mounted on top of my steering column and monitor coolant temps. Before Evans my average operating temp was about 202F. With Evans it is about 198F.
Though his name is not on the product, I believe that John Fitch (check Wikipedia) had a lot to do with its development.

 

I use Evans in my street legal Drag Car, and it's what I will always use in it. I haven't put any in the XKR yet, but I certainly will when the current coolant ever has an issue.

 

I have no personal opinion or experience with Evans, other than doing a little research into this myself about a year ago. But there are a myriad of "oil thread" type debates in various forums regarding Evans running the gamut from love to hate.

I'd simply suggest reading through those to see when and if Evans makes sense for your car.

One interesting and often cited analysis was performed by Norosion, which admittedly is an indirect competitor to Evans that offers corrosion inhibitors meant for traditional water based blends. But it is an interesting read.

No-Rosion Products Technical Questions and Answers

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I started using it mostly to try and lower the operating temperature along with an attempt to stop blowing out freeze plugs when winding it up to 8000 RPM. It did both. Plus, Evans on a drag strip is preferred because it isn't slippery and difficult to clean off the "groove" is some happens to drip or dump. It really is kind of fun to have someone walk up to my running car and I can take off the radiator cap at full operating temperature with zero expansion.

 

Coolant temperature is regulated by the thermostat so any change is operating temp is not due to the change over to waterless coolant. It is also well known and admitted by the manufacturer that this product has vastly inferior heat rejection properties, a critical issue for cars with little or no margin in the cooling systems.

Given it's cost, inconvenience during installation (and throughout it's useful life) balanced against minor or non-existant gains, it makes little sense to use such products.

It's just another case of a 'miracle' solution in search of a problem.

'Water wetter' is simply the same package of additives already present in all conventional coolants. Adding more of these elements to coolant does nothing.

 

I've been reading up on Evans too. Their website says you should expect the temperature to increase with their coolant. But since it has a higher boiling point, it's not a problem. The big improvement to me is the lack of pressurization. In an old car, this might help avoid problems since it reduces stress on the whole coolant system. But you have to remove all the water in the system which can be difficult to do. You can buy their flushing fluid (at $30+ per gallon). If you leave more than 2-3% water in the system you have to start all over again with new coolant. And I think the cost is closer to about $45 per gallon or more for the automotive coolant. I really don't see a lot of advantage to using it on a modern car that doesn't have any problems.

Govtslug

 

Leakage problems on old cars would be from old tired components that are at the end of their service life. Reducing system pressure is simply a temporary band-aid, not a cure. If and when they finally let go, in addition to losing all the expensive coolant, a fresh supply would be required. This would mean carting around a spare supply in the trunk or sticking close to home at all times.

Much simpler to keep the cooling system in good condition and use inexpensive conventional coolants. Water can be used as a top-up in a pinch, not possible with the Evans type.

 

Service life of coolant parts is often cut short by pressure leaks. Absence of pressure is an advantage of Evans.
Removing all the water from the system is not a big problem. At one time, years ago, Evans stressed removing 100% of water. Now, they have backed off a bit.
Whatever, I removed water the Evans-approved method - drain, flush, etc. There was still some water in the system. I left the coolant recovery tank cover off, and over several weeks most of any residual water "steamed" off. This required adding some Evans coolant as makeup. Took a qt. on a 5800 mile trip, and added less than a cup.
Assume a worst case of total coolant loss while stuck somewhere in the countryside. Usually, such a total loss is preceded by some leakage, so assuming you did a check for visual leakage chances of a sudden total coolant loss becomes less. What to do - just add plain old water. That will get you home and is compatible with Evans. You'll have to go through the Evans procedure once you get home, but you will get home.
I don't think Evans makes a lot of sense it you intend to keep your Jaguar less than five years from new. But, now that the XK/XKR are out of production many of us will be keeping these cars much longer.

 

Switching to waterless coolant on an older car still doesn't get around having already aged components. Pressure is only one factor in their demise and probably not the primary concern.

In your limp home scenario, you mention refilling the system with plain water. This would also require restoring the pressure cap to ensure that the system comes up to normal pressure. Unless the root cause of the leak has been properly repaired, there is a greater danger of coolant loss and engine overheating.

Again, these products sound great in theory but not so much in the real world.

 

Back to the real world. Most folks will retain their pressure cap, loose or tight (on the coolant recovery tank), so after adding water just tighten it once again and the system is under pressure.