Good question because the latest Alpha3 rules (including UK rules in blue) do not address this.
Look at the original rulebook, though:
Page 32, under “Global UK rules”
“…UK units purchased and placed on the Pacific map can only be bought with IPC’s generated on the Pacific map. Likewise, UK units purchased and placed on the Europe map can only be bought with IPCs generated on the Europe map.”
Seems clear that London can’t buy bases on the Pacific map (except on Yukon or BC) and Calcutta can’t pay for bases on the Europe map (except Western India). Even if the other capital is down. Krieghund will say if this is wrong, but I’m 99% sure