Condition is everything with 80's rifles. The going rate for a top end rifle in mint (possibly boxed) is stuck at about £250. A more desirable rifle difficult, to find, up to a tad over £300 unless it was a very expensive rifle to start.
Your rifle is collectable and used, so not mint, however it is in very good condition and a desirable rifle. I would say worth about £230 and the scope might have additional value (a few named scopes have real value, most don't but just add to the sale). A well used Khamsin is worth about £160 and a rough one £140. These prices are a premium above other equivalent rifles of the time.
These rifles looked great but the action and trigger were disappointing. They never sold in huge volume as they were expensive in their day. Air Arms cut their teeth on them, the rest is history.