Airborne transmissions of any kind are specifically prohibited in all UK amateur radio licences.
The official information on radio controlled models is OfW 311 - Radio Controlled Models
. Section 11 of the page discusses amateur radio, airborne operation and licence exempt video transmission. When we checked in February 2015, airborne transmissions were legal from compliant equipment—see links from ofw311
—operating on the 2.4GHz band with a maximum power of 10mW EIRP and on the 5.8GHz band with a maximum power of 25mW EIRP. Using a medium gain receiving aerial and a sensitive receiver, a video range of around a mile should be quite possible (but note that it is generally illegal to fly your model beyond unaided visual range). Further useful information about legal first-person view (FPV) video
is provided by the British Model Flying Association (BMFA).