Is the use of radar detectors allowed in the UK?

Yes - Since 29th January 1998, it has been legal to own and use a radar/laser detector in the United Kingdom. In summary, passive radar detectors that simply warn you that they are receiving a radar signal are NOT illegal, but the use of equipment that then transmits a "jamming signal" back to the radar device IS illegal and this link has more details.

However, forthcoming Government legislation, could mean that the use of such devices will become unlawful and this link has more details.