A frequncy shifter can be used. This is a small device which is plugged in between the aerial and the aerial socket on the radio and 'shifts' the frequency by a prescribed degree. I had one in my Japanese Subaru as th factory radio used Japanese frequecies, which go from about 76-99 FM or something like that, but considerably different to that of the UK. The results were OK, though you have to think a bit when looking at the frequency dispaly, as that cannot be changed. RDS also won't work. What I would suggest is to try looking for an American car importer somewhere in the UK, as they will face this problem and probably install one of these devices into all their cars. If you can't find one of these, speak to a Jap car importer (there are milions everywhere these days - check the autotrader under Subaru or Mitsu or Toyota) who may be able to tell you where they get theirs from, then follow this up. I'd imagine Maplin electronics will have something - they always do! Once you have the device, pull out the stereo, pull out the aerial, plug the device into the aerial socket and plug the aerial into it.
There may be some newsgroups relating to car modding that might help with a reprogram, but I'd think it would be tricky. Google the model number and trawl through the repsonses (may take some time)