The initial planned uptake of F35, linked to the carriers, is 48 airframes. On Typhoon, given the immediate pressure on numbers, wouldn't the simplest route be to retain some T1s in the shortish term to stand up an extra sqn or two and use them purely for AD? That could serve as a temporary measure pending adding an extra sqn or two of F35 from the mid 2020s, at which point the T1s could go leaving a force structure of say 5 Typhoon sqns (T2s and T3s) and 4 sqns of F35.