(3) OFCOM may by regulations exempt from subsection (1) the establishment, installation or use of wireless telegraphy stations or wireless telegraphy apparatus of such classes or descriptions as may be specified in the regulations, either absolutely or subject to such terms, provisions and limitations as may be so specified.
Mobile phone handsets are exempt under the terms of The Wireless Telegraphy (Exemption) Regulations 1999, SI 1999/930, although I think that the exemption has now been transferred to a later 2003 Regulation.
Basically anything using RF must be licensed unless it has an Exemption. So you don't need a licence for a PMR446 transceiver, a Tracker system, a WLAN router etc - or even a CB radio these days. But you do for aircraft emitters or marine transceivers, for example. The Exemptions will stipulate conditions of use.