Comreg watchers will be interested to learn that it has today issued the catchily-titled Prohibition of Sale, Letting on Hire, Manufacture, and Importation of Wireless Telegraphy Interference Apparatus Order 2011. The statutory instrument does what it says on the tin, and makes it a criminal offence to import, sell, etc. jamming devices - in particular mobile phone jammers.
I'm not sure what prompted this action now (growing numbers of cheap jammers being imported via Hong Kong sites?) though it does plug a gap which was recognised as far back as 2004 when a Comreg consultation on mobile phone interceptors pointed out that the use but not the sale, etc. of jammers was illegal (Consultation Document | Response to Consultation).
Incidentally, there is an overlap here with offences under the European Communities (Electromagnetic Compatibility) Regulations also, as by their nature jammers cause excessive electromagnetic interference and so could not be lawfully put on the market.
(h/t Ronan Lupton)
Although it does specify mobile phones in the explananatory note, I'd say the issue of GPS interference had a very large part to play in the development of this legislation. See this PDF on the Irish Lights website for more, it's an interesting read: