Plusnet will have acquired a new range of IP addresses which was previously either unused or used in another country.
It hasn't yet propagated round all the necessary places. The BBC do not allow iPlayer to be used outside this country so are rejecting the OP's IP address when they check the location.
If he is on a dynamic IP then I suggest disconnecting for 30 minutes or more and try again. A quick re-sync could easily get the same or a close address. Even 30 minutes might, but it's worth a try.
My broadband basic info/help site - www.robertos.me.uk
My domains,website and mail hosting - Tsohost.
Internet connection - O2 Standard.