Since you are from the UK, you might get issues trading with leverage. This is an issue with any countries including the USA.

If you are depositting fiat then you need to use an exchange like Binance but most will only let you trade spot. There are some unregulated exchanges that might let you trade but you will need to deposit crypto, cant deposit fiat. And I would avoid those because they can run off with your deposit.

Like others said, you can try using a DEX. But with a DEX you need crypto somehow, so you will need to make an account, buy some stable coins and withdraw those and then you can try a dex. However keep in mind if leverage trading is restricted in your country, many Defi platforms will have a ToS disclaimer and prevent letting you trade based on your ip address.