David Frost With some minor exceptions it is now an offence to
manufacture, import or sell a realistic imitation firearm or to
supply one to a person aged under 18.  It is not an offence to
possess, give or receive one (if you are 18 or over). You are
therefore in the clear with your gun but when it becomes surplus
to your requirements you must give it away, not sell it.
Imitation firearms are not regarded as being realistic if they
are one of seven specified bright colours.  For this reason you
will find that new versions of the gun you own come in a range
of snazzy colours. You are not allowed to carry any imitation
firearm in a public place without lawful authority or good
reason.  Dog training or proceeding to/from a training session
count as good reason, even if you stop at the supermarket on the
way.  Having it in your pocket simply because you’d forgotten to
put it away would not be a lawful excuse.