Some seats will fit, but they are only just...
Britax Eclipse Si certainly does but it's forward facing. We had an M&P travel system before that, which was rear facing and it JUST fit with zero slack. Best bet is to take the truck to Halfords, Mothercare or whoever and try before you buy. You will be limited!
FWIW, Mandy's old Ford Escort had the same trouble - half the seats in M&P were counted out by that. The newer cars seem to have longer belts.
You could try to source a small extension belt (sort of female to male) about 2 inches long - could give you just the extra you need without getting in the way of the way the belt loops.
Another option (and the route I almost took), remove the seat belt and put a longer one in!
You could try to source a lap-only car seat, but I think these are only available for forward-facing. My Britax allows lap only (which means I can carry Charlotte in the 101 and Series 2 (when Mandy isn't looking...).
Forward-facing seats inherently need less belt, but are no use until Junior is at least a year old. Don't go by the age, BTW, neck strength is the most important factor and varies a lot from infant to infant. OOI, Volvo don't sell forward facing seats at all!
Good luck.
If you find one that fits it might be worth adding to the FAQ as a useful reference. Phil....