I'm guessing it really depends on what parts of GCSE's you struggled with. For example there isn't a lot of work in GCSE thats carried on into A level, you won't find any protractor/compass work but you will find much much more sin/cos/tan work, so if you were bad at drawing you'll be fine. Also you take applied modules so if you like the "handling data" aspect of GCSE you'll quite like Stats or if you prefer the algebra side (quadratic formula, factorising) Mechanics is the way forward.