What is the best site for this in the UK?
Just like everyone said, it just depends on what coins what you are trying to invest. You might want to check that in coinmarketcap to make sure you have best liquidity on your assets. Can't really recommend anything more than that if we don't know what you are trying to do. Make sure to do any research before deciding anything on trading, it's really fatal for your journey.
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.
That's pretty much a decent capital to start with, the only problem is how do you manage to maintain your trading journey and your investment.