Looks like a Venom Vantage with a 10 inch silencer.
Are there 2 small holes on the underside of the silencer towards the breach?
Value depends how much someone wants it, i have bought 2 venom 77's in the last few months in good nick.
I would say between £300 & £400 because it's .22, tends to be a bit more in .177 and in A1 condition.
Hope this helps