Option 3: If at first you don't succeed, try, try again. What do you have to lose?
It'd be nice to have more information to go on... like how fast were you going, did it lock up with screeching sounds like spun bearings or locked up piston, just quit at slow speed or idle....
The fact that it has iron cylinders is in its favor and if an automatic, the torque converter is more forgiving than a stick/clutch.
Pull the plugs and shoot some penetrating oil down the cylinders. Try the starter, over and over again. See if you can get a breaker bar & cheater on the crank and break it free.
Option 4: I may be old fashioned but I still repair/rebuild my own engines. (I'll bet I'm older)
Option 5: Buy another car. That last two beautiful XXK8 convertibles we bought were each just over $2,000.

Have to agree with the above?
Might as well give it a shot and see what happens.
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Yes, remove all of the plugs like jrnsr had suggested, and squirt some penetrating oil down each cylinder. Allow the penetrating oil to soak overnight before trying to break the crank loose with the breaker bar. Perhaps a pipe over the breaker bar might offer some extra torque in getting that engine free as you got nothing to lose at this point.

Thanks, I'd love to try option 3, but at the moment, the car is 200 miles away and I do not have access to a vehicle that would tow it. Working on that. But I'm a teacher and it is standardized testing week, so I'm not in a position to do anything this week.

More info: We were cruising at 70 mph, she had just passed somebody so the car was momentarily up around 90 mph, . The light came on and she said a voice in her head told her to pull over immediately, but she didn't and we pondered what it could mean and what to do. Then the light went out. Then noise started building up and within seconds the engine froze up. We pulled over and popped the hood (sorry, bonnet). I could see that the coolant was boiling out the cap, but the temperature gauge still read normal. There were no traces of oil drippage under the car. Days after when the mechanic looked at it, he said that there was some oil in the crankcase. I don't know how much, but a friendly person had stopped and offered help. He had a bottle with about a pint of oil in it that I had put into the engine.
We always knew that there was a slow oil leak, and we also missed a scheduled oil change a few months ago.
The car had just been in the body shop for work on the undercarriage, but nothing mechanical was done to the engine that we know of.

Sorry to hear this story. We really can't expect everyone that drives older Jags to be enthusiasts or mechanics. As an enthusiast I realize that my Wife is not going to be too empathetic to an older car, so I had to be sure that the car was always in tip top shape if she was going to drive it. A couple of years back we came across a very nice 2005 XJ, it was one of my favorite color combos, white exterior with tan leather. The price was right and it drove really well, but the parking sensors would go off occasionally. My Wife was so enthralled by the car that she stated that she would want to drive it if I bought it. That really hit me hard. We all know that these later model Jags are very sensitive and prone to problems and sudden failures, like of the cooing system. They cannot survive being overheated and they must be stopped immediately if there is any indication the the motor is over heating or a strange noise or lack of power is experienced. I knew that my Wife was like most drivers, she isn't going to stop immediately, she is going to continue and try make it home, or comes to what appears to be a very safe place. By that time, it's usually too late for the motor. I knew that a car like that XJ could not be a family car driven by the Wife or others so I passed on it. I tried to explain the issues to her but she just didn't get it. Hobby cars are for me, she can drive the late model minivan, I don't care what happens to that.