Bands that arem't real? That's a new one on me!
You raise an interesting point. Licence collecting societies exist and are authorised under Sections 116 to 123 of the Copyright, Designs and Patents Act 1988), to gather royalties on behalf of their members. But not all artists are members, although of course the collecting societies don't always make this clear. A recent test case before the Copyright Tribunal revealed that radio and TV stations which broadcast mainly music have to pay a licence fee ranging from 8% to 25% of their annual revenue. Unfortunately I can't provide a link to the relevant judgement* because the ipo.co.uk site is down at present having suffered a DDoS attack earlier today.
Anyway, returning to your question, if you are only broadcasting music from bands which haven't authorised the collecting societies to collect royalties on their behalf, then you do not need a licence. The main licensing society for recorded music and music videos is Phonographic Performance Ltd (PPL)
http://www.ppluk.com/.
However bear in mind that someone will own the copyright in this music, even it is entirely generated by a computer, since there will be a human 'directing mind' which wrote the software. Therefore you need their permission to broadcast their music, unless it has been released under a Creative Commons licence.
*Update. The Copyright Tribunal site is now working again. The judgement I mentioned can be found in .pdf form at
http://www.ipo.gov.uk/csc-decision.pdf