I’ve done a bit of research on this as I wondered the same myself. Starting from the assumption that you want the overall diameter to be about the same as original to preserve speedo\odometer accuracy my summary is:
– 235 wide should (must?) fit as Jaguar specified this on later cars although I’ve never actually seen an xjs with these tyres on it as they are such an odd size in 15″.
– this width fits all 4 wheels the same offset
– I think 245 width can also fit but it would be very close
– wider tyres can be fitted but you need different wheel offsets front to rear (which is what TWR did to put 255s on the rear I believe)
– standard offset for Jaguar xjs wheels is 28.5mm
– wider tyres on the rear will hit the inner mudguard, on the front the inner steering\suspension somewhere (hence the need for different offsets). Rear wheels need to be moved inwards (larger numerical offset number) & the fronts outwards
– don’t forget that xjs’ also need a wheel with enough centre hub diameter hole to fit the front
– this is all theoretical, I run standard Jaguar 16″ wheels on my car!