I had a go in the Catherham @Brands and had the AI at 105% on medium aggression, that usually gives me a good challenge. But for some reason i was several seconds faster in this combination. I switched to MRC2000 at Outlon, same story. Even att 125% I find the AI being slow but it only seems to cars/tracks in the brit pack DLC.
I've experienced the same as you Martin. At 120% I'm still considerably quicker than the AI on the Brit pack cars. However all other cars seem fine