depend on what country you are, but you can also buy with BTC with Cash from Bitcoin ATM
you can search on google Bitcoin ATM in London
otherwise if you want to buy online I would suggest to use Binance
Not all countries that have bitcoin ATM machines, the easiest way to buy bitcoin if ATM is not close is by using exchange, the criteria to follow will be listed in the preferable exchange it chose, using binance is good but you need to complete your KYC verification in exchange if you wants to be buying and selling of bitcoin