A licence is not required to use a radio receiver or scanner as long as it is not capable of transmission. It is not illegal to sell, buy or own a scanner or any other receiver but it must only be used to listen to transmissions meant for general reception. The services that can be listened to under the definition of general reception are:
a) licensed broadcasting stations

amateur and citizens' band radio transmissions
c) weather and navigation transmissions
It is an offence to listen to any other radio services unless licensed to do so, this includes listening to illegal radio stations (pirates) by virtue of the fact that they are not licensed radio stations.
It is an offence under Section 5 (1) of the Wireless Telegraphy Act 1949 to use a scanner to listen to any other radio service (this includes any aeronautical, maritime, emergency service, armed forces, business or mobile telephone transmission), without the authority of the Secretary of State, or the Commissioners of Customs and Excise.
Not sure what the penalties for the offence are, but they definitely involve the siezure of your equipment.
Turning up, taking pictures at fires isn't on really, especially if the photographer is getting in the way of the firefighters. I know if my house caught fire and someone turned up taking photos, i'd smash their camera over their head!
Thank god/allah/budda etc for TETRA!