I don't really follow AMD as a CPU manufacturer, so I had to look up the 3650. Seems it's often compared to (and in the few benchmarks I saw, often pipped) the i3 2100 from Intel's camp. I doubt you'd be getting the full potential from a 7850 with that CPU, and it's almost twice the £100 budget, which makes it hard to recommend for me.
The 6850 is more powerful than a 1GB 460 from the benchmarks I looked at while trying to pick out a card for a friend on a similar budget, which should mean it's enough for most games on decent settings at up to 1080p. I used to run a 768MB 460, and it coped pretty well with most things I threw at it.