The relevant authority used to be the Radiocommunications Agency but that has now been absorbed into the all-encompassing Ofcom. There is an overview of the law here:
http://www.ofcom.org.uk/codes_guidel...thelaw?a=87101
On the Radiocommunications Agency site there used to be a good document that listed all the frequencies and powers for various purposes that were permitted without a licence. Off the top of my head I remember you can transmit data in bands at around 173, 433, 860 and 2400 MHz. You can transmit audio at 866 MHz (cordless headphones) and video at 2400 MHz (videosenders). Transmitting outside those bands, or using a band for the wrong purpose is illegal. Special penalties are reserved for those working in a broadcast band (such as 87.5 - 108 MHz FM) as they are regarded as pirate stations.
I suspect, however, that any iTrip is unlikely to cause a problem but, if someone does complain to Ofcom, they are bound to persue it.