In October 2007 the law around BB guns and Imitation Guns changed. The Goverment decided that guns that look like real guns would be deemed as illegal.
What this means is that if you own a BB gun or a gun that looks like a real gun and you take it into a public place, then you could be arrested for possesion of a firearm with the intent to endanger life. This could possibly mean that you may be prosecuted and sentenced to 5 years in prison.
However, not everyone has got a BB gun - some people have imitation/replica guns. These can be guns that are used as models or guns that are put on people's walls or used in TV shows. These guns are OK in the privacy of your own home, but as soon as you take them into a public place you then commit an offence.
Guns such as these have been used to commit serious crimes, such as robberies and it is because of these offences, the law changed. If you carry one of these guns and you are stopped by police, the officers will not know if it is a real gun or a toy or a fake. This could result in you being bitten by a police dog, tazered or worse killed by armed police officers.