I have been busking at the southbank when funds have gotten low since moving to London a couple of times this year..it's varied in success...and go early, there's a few regulars down there who get some good spots and if you leave it til the afternoon you could miss out and makes all the difference as to sound and people traffic to listen..
Dont expect money, you cant predict it, but have fun! I do and go every now and again when I'm broke or the weather is ok!
I dont know if you need a license or not...but nonone said anything to me so I will be back!#
Good luck