I suggest you contact Tanya Batteries.
They provide an excellent, and safe, mail order service.
Assuming your bracket has not been modified they will be able to supply the 'correct' battery, at a very effective price.
Batteries with higher CCA are not necessarily any larger, but care is needed.
Mine, a Yuasa, was easy to fit; but you will almost certainly need to reset the windows, and maybe the moonroof too.
Both an easy process (assuming they work correctly)
There is always Halfords, or Kwik Fit who can/will test your current battery first (without obligation).