It’s possible, it’s always a cat and mouse game, but since no one actually owns the currency, tracking who paid who is still a challenge for them.
They can track exchanges or cloud wallets for instance, but I’m not so sure about peer to peer trading, and I’m not sure they would consider it worth it for low volume trading.
UK has quite a lot of freedom comparing it with many other countries, so even if they manage to enforce those regulations, I don’t expect them to be too much of a hassle(even if in principle I’m against it). That kind of country doesn’t seem to have the kind of restrictions that makes you want to actually use bitcoin. In that regard I guess most people buy bitcoin because the price always goes up and they want some quick money, not trading.
But countries with more restrictions to foreign trading and currencies are less likely to have the resources to enforce regulations on crypto currencies, even if they pass a law(pretty much all of Latin America). Their incompetence is what drives them to restrict foreign currencies so people don’t run away from their debased currency in the first place