It's illegal already, so far as I can see. It's just that they don't bother to do anything about it except when they feel like it. Morris Dancing in pub car parks, it's the same story.

Or rather the trick is, it's not illegal to do it (though for busking if the police tell you to move on and you don't it's likely to be seen as a breach of the peace, which is), but it's illegal to organise it or to permit it, if you are a proprietor of the place where it's happening, so long as that is a place which is open to the public. For the street I imagine the proprietor would probably be the local council, which is unlikely to prosecute itself. In a shopping mall, for example, or a shop doorway, it might be a different matter.

The expectation is that the proposed "reforms" won't change those things; what they will do is make it impossible for pubs to use the two-in-a-bar exemption as a loophole for allowing a session with more than two, because they won't be allowed to permit even a single person to "perform" unless their licence specifically covers it - which it probably won't. The option of just keeping your head down and doing it anyway won't be on any more.