Is my .22 blank firing revolver an illegal gun?

Legal
DAVID FROST says:
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.