Leena, you don't say where you're from, but if you've been booked by people you haven't known about before to play a concert in the UK for a fee of £20,000 then you must be a super star playing mainstream music. In which case you'd have played in the UK before and know the ropes about getting a work visa.
£20,000 is an enormous fee for a musician. The gig would be well publicised and you would be well taken care of by the promoters. It doesn't sound as if that's the case. If you are a superstar, then tell us your performing name and we'll all cheer from the rooftops for you! If not, call the British High Commission yourself to find out what's happening (or not) with your visa.

This is a scam. If something sounds too good to be true, then it is. If you have personal details for any of the people you mention in your post, then give them to the police where you are and tell them that they're involved in some kind of international fraud.

I've had similar correspondence twice now - read the rest of this thread and you'll see how annoyed it makes me.

Anne