I've never bought BTC from a person with cash, but you can do so through
localbitcoins.com. Alternatively, and I prefer this method, you can pay for BTC through a BTC ATM. There were four in Brighton (one of which transacted in Monero), but I haven't used them since the Summer, when wearing muzzles in shops became mandatory, so I can't guarantee they are still there.
They were; SV News, 54-55, Kings Road, Brighton BN1 1NA
Arcus Vapor, 19, Brighton Square, Brighton BN1 1HD
Social Board Brighton, 21 St James' Street BN2 1RF (also deals in Monero)
AM PM Store, 59, London Road, Brighton BN1 4JE
I hope this helps.
Regards.
Bri