No, the GPU in a laptop isn't replaceable. If you want a good GPU in a laptop, go at least for the GT 640M as it features more processing cores and you're able to overclock it to GT650M level.
Probably not. You would need to wait for a sale. Not sure of any holidays in the UK other than Christmas so if there is a major holiday coming up before Xmas, then just wait and scour the internet for any deals.