I had the same issue on my 1995 XJR where a low battery voltage (10.5V) would not allow me to start the car.
If battery is newishbut charge is low for whatever reason, try the following:-
1) Ensure battery has been fully charged because the ignition key excitor coil appears to be voltage sensitive, thereby affecting the immoboliser system and the car won't start (starter motor won't turn over).
2) With a multimeter measure battery voltage with ignition switch "off". Should be =>12.3Volts
3) Hopefully you'll get the car started and thenlook at multimeter. At idle rpm, the voltage should rise to over 13.5Volts, generally in the region of 14-14.5Volts as the engine rpm rises. If this is the case, even after switching "on" all other electrical loads (lights, rear demist, A/C etc), then your alternator is OK.
4) If however the voltage reads <13Volts, especially when heavily loaded and running @ ~ 2,500rpm, then the alternator is faulty and won't keep your battery fully charged over time.
I discovered two things with my XJR to cause my starting problems: a) the alternator voltage regulator was faulty thereby not providing the necessary charging current over a period of time and b) the dash light's remained on after turning off the ignition (a small control module fault, corrected by simply turning the dash lights off at the dimmer switch on the steering column). These dashlights, even at such a low wattage, drained an already low battery overnight, to a point where the said ignition key excitor coil (tell's the immoboliser that a key is present in the ignition switch) would not "energise".
After charging the battery on a trickle charge over 24 hours (don't ever boost charge a battery, always trickle charge it) and afterreplacing the alternator, everything was well again.