Casio FX-991EX Engineering/Scientific Calculator, Black (European Version) https://www.amazon.co.uk/dp/B00ZZ93346/ref=cm_sw_r_apan_glt_fabc_8PFJBRQTW4KAMV3PFESC
The 991ex is the minimum you need. The GCSE scientific calculators are no good for the A level as they do not have all the stats functions required.
The cg50 is nice, but expensive. If you're doing FM, some schools will loan one but it's not common. 991ex will do everything you need.