Coinbase Pro seems to be the cheapest place where you can buy Immutable X (IMX) in the UK (max 0.5% fees). However, this isn’t a beginner friendly option.
Another option is CoinJar, which has a great user interface (on web and mobile) which is intuitive and perfect for beginners. Fees are slightly higher though (1% if using a UK bank transfer).
Read on for more details.