I'd guess that the main roads wont have changed that much, but the village / town details will be much improved. If you're feeling brave work out where the waypoints are in V8 eg hotels and then use V5. The last bit on the road won't be exact but at least you'll know where to go, if that makes sense.
The other, better, option IMHO is to buy one of the 256 mb cards on fleabay. That way you'll probably get to whatever port on the 128, and use the 256 on the other side of the channel.