The post by Yorgos Papavgeris is slightly misleading - the licensing issues he refers to are to do with local authority licenses for the performance of music and other events. Copyright issues in the UK are dealt with by the Performing Rights Society. There are lengthy threads on here if you search.

As a performer, you are covered by the PRS licence which the venue must have. In theory, a licence is required for other public places, hence the occasional shock-horror when PRS demands a licence for a radio played at work.

If someone records your performance and then posts it, I don't think the owner of the copyright would have any comeback against you - they get their royalty from the PRS licence for the venue where you performed. They would have a claim against whoever posted it. I think you might also have a claim against the poster, as you would own the copyright in your own performance, but not copyright in the material.

The fact that copying music has been going on for decades doesn't alter that it is wrong, both legally and morally.