If in Doubt, Go Retro
In my opinion, there is no better swimsuit than Esther Williams's retro one-piece, and believe me, I've done extensive research on this. You may baulk at spending £70 on a swimsuit, but if you can save up, you won't regret it. I own this in two colours and I also have the matching bikini, and the moment I put them on I feel like a pin-up girl. They're good quality (lined in the front to avoid any see-through postswim moments), and the clever gathers hug the body perfectly. ModCloth sells extended sizes and ships to the UK, but note these are US sizes so you'll need to subtract 2 or 4 depending on the kind of fit you like (they run fairly small but a US 10 still fits me fine). You can also buy some colourways from For Luna in the UK, which is where I picked up the amazing lobster-print version.
ModCloth sells extended sizes and ships to the UK (around £70)