One would hope that by the time the F-35 comes on stream for the UK, the Typhoon will have begun receiving the upgrades that will turn it into a full all-rounder also. If that happens, the RAF will in theory have two 'omni-role' (to coin the French term) fast jets to choose from.
Countries such as the Norway, Netherlands, Denmark, and Turkey will be fielding the F-35 as their sole type so you'd hope for their sakes that it has a more-than-credible air defence capability.
Actually, despite what I said earlier half of the nations currently singed up will have a mixed fleet, so interesting to see how they task the F-35 in relation to their other assets. Until I heard the RAF comments, had always assumed it would be roled a bomber, with the 'other' type (Typhoon/F-15/F-16 etc) acting as the fighter, but now not so sure.