What is the best site for this in the UK?
It's hard to say what is a best exchange for you because to choose an exchange, you have to consider many things: KYC or non-KYC, Taker, Maker trading fee, withdrawal fee.
I am not sure about UK, you can find some from exchanges for Europe.
https://www.cryptowisser.com/exchanges?continent=EU&coins=&fee=2.0If you don't like KYC
https://kycnot.me/I am only going to start with $200 roughly to see what I can turn it into over the next few weeks and months as the market is starting to get very volatile so could be good opportunity to make some extra money which would then mean I can invest more into my long term plays.
I will go frankly with you, trading is difficult and it's hard to get rich with trading.
Your capital for trading is smal, $200 and I will use it for investment to layer 2 projects like Arbitrum or Optimism. If these projects successfully make its bull run in 2024 and 2025, chance for x10 or x20 with their tokens is possible. It's a better choice than trading.