"if you don't control your private keys, you don't have any bitcoin"

Coinbase is a centralized wallet, meaning, if they wanted to lock up your account for some random reason, they can. If you're only going to invest like £200, then use Exodus wallet[1] instead, so you can hold most of your coins on a single wallet. If you're going to add up some funds in the future though, I suggest purchasing a hardware wallet like Ledger Nano S[2] and Trezor[3]. I can't stress this enough, a hardware wallet is an essential in terms of securing your investments.

You can only buy bitcoin, litecoin, and ethereum through Coinbase. To purchase ripple, send some BTC/ETH to an exchange like Bittrex[4] and or Binance[5] and exchange those coins for some Ripple.

[1] https://exodus.io
[2] https://ledgerwallet.com
[3] https://trezor.io
[4] https://bittrex.com
[5] https://binance.com