If you're going for your ATPLs you can even consider getting an FAA PPL. Within limits this will allow you to fly a G-reg within the UK anyway, and for your JAA CPL, *any* ICAO PPL (including an FAA PPL) is good enough.
1. This mainly depends on the weather and personal ability. Make sure you have the ability to extend with a few days if required.
2. It can be better, it can be worse. Depends on the comparison. It will be good enough to obtain your PPL though, and after that it's up to you to learn to fly properly. Your PPL is a license to learn, after all.
3. No you will not be. But if you go for a one-to-two hour flight with an instructor in the UK and expand on that slowly, you will have no trouble adapting.