I would suggest the A300 as a replacement for the Nimrod R1. There are plenty around and, if greater range is needed, could also be fitted with additional center tanks. Although it would have an endurance of around 9 hours with the normal 55 tonne fuel load.
Elbeflugzeugwerke have been doing bespoke A300 freighter conversions for some years - I understand that Filton also has expertise in this area.
Additionally, the A300 is smaller than the A330, so perhaps easier to base at most RAF aerodromes. Overall length 177 ft, wingspan 147 ft, so in the VC10 size category, more or less.