If TFL is anything like a few months ago there isn't a 'law' requiring mask usage, it's just a condition of carriage[1]. Also, they're enforcing it with enforce

If TFL is anything like a few months ago there isn't a 'law' requiring mask usage, it's just a condition of carriage[1]. Also, they're enforcing it with enforcement officers. There may not be many and they may do a poor job of actually challenging people but I've seen it at stations a few times.