Phoney
Granted, Waddo might be a bit short on certain days. But gas and go from Heathrow, Gatwick, Brize or Boscombe and you should be able to haul out maximum gas in most UK weather for a "UK only" op. Don't forget Offutt is only 11,700ft and is nearly 1,100ft up and the RJs, I believe, lift out maximum gas.
I agree, my figures are probably slightly off, but I'd be really surprised if you could not get 12 hours out of an unrefuelled RJ. Cruising at around 370kts GS that would give you ~1800nm out and ~1800nm back with 2hrs on station at endurance and a divert fuel for something close.
These are all fag-packet numbers having flown a similar type with the same engines. I have never flown an RJ and do not know of its LIMFACS, but I doubt I am wildly out with my estimation that even without UK AAR support it is likely to be very useful unrefuelled.
LJ