FL490 was a limit imposed by the MK17 reg as it was a pressure breathing gadget only. RAF rules required some sort of pressure suit to go above that. Prior to conversion to tankers, the B2s and SR2s had the facility to run a pressure jerkin - a partial pressure suit which covered the torso only and allowed the wearer to breathe the high pressure oxy supplied, as well as stopping the wearer's lungs going pop!.
The B2s could get to FL630, the SR2s to FL670 apparently. The K2 (tanker) had shorter wingspans, uprigged ailerons, refuelling pods and underwing tanks on - all of which reduced it's max alt.