Martin G8JNJ has brought to my attention a little known, to me at least, OFCOM concession that appears to allow licence exempt operation under certain conditions in many, if not all, parts of the radio spectrum for research and development purposes.
Although OFCOM will grant Non Operational Licences (these were once known as Test and Development Licences) and charge you £50 a year for the privilege (!) to test new equipment or carry out specific research, they do NOT require a licence if emissions measured at a specified distance are below certain levels (supressed radiation conditions). See http://licensing.ofcom.org.uk/binaries/spectrum/non-operational-tech-licence/ofw357nonopguide.pdf for some guidance notes on this.
My own interest is to research earth-electrode antennas in the frequency range between 8.97kHz (where I already have done some tests) and 137kHz (where I have also tested this antenna structure). Using WSPR and other weak signal techniques, quite a lot of useful work can be done with low uW EIRP levels. It would be interesting to see how such an antenna structure behaves at say, ~40kHz and 73kHz. It may just be possible to do tests at moderate ranges (in the far field) without a licence. Now wouldn't that be interesting?