Andy,
I understood that you needed a CAA license/permission for any form of commercial work involving a UAV.
I'd read this page:
http://quadcopter.heliguy.com/Certification-and-Law/
and then:
http://www.caa.co.uk/default.aspx?catid=1995
I believe the BMFA insurance only covers you for "hobby" flying accidents, so appropriate cover would be required if undertaking any commercial work flying over people or property, even if unpaid.
You may have your own professional indemnity and liability insurance, as I do for purposes of my business, however I'm fairly sure the terms will not cover for a specific quad flying accident, hobby flying or otherwise. Maybe something to check with your broker/agent?
I also think for commercial flying insurance purposes you need a certificate of airworthiness for the quad, again which implies you have to get it built from a reputable supplier, rather than a "home" build. The new flight limit features of the Naza v4.00 software is a requirement for certificate of airworthiness, previously only available in much more expensive flight controllers.
UK specific information is a bit vague, intentionally so to get you to engage with a reputable UK UAV dealer, perhaps?