P2P transaction could increase but you won't be able to spend those coins anywhere.
I don't think that's the case. It takes only around 35 minutes to cross La Manche via Eurotunnel. Once you're across, those coins can be spent pretty much anywhere in the EU. As for p2p trading, I'm the regular user of both bisq and bisq2, and I often see UK based traders actively buying and selling BTC there. So, these kinds of transactions are definitely hot among Brits.