The issue with Paypal is that Paypal is reversible. So its very risky for the individual who is selling the Bitcoins. Paypal used to be a popular option a few years back when there was less exchanges, and everybody only did Paypal by escrow and it was fairly safe.

However these days there is no reason to use Paypal since there are many exchanges. And if you need the cash right away, just go to a Bitcoin ATM which sells GBP for BTC. The fee is usually high 10% but its more convenient for some.