Leeming - a little further north than ideal, given that most displays are in the south of the UK. But a good long runway and plenty of available space, with only Tatty Ton and the nascent T1 AFS being in residence. Of the 3, the only one which doesn't require departures and arrivals over a major road. Teesside/Durham or whatever it calls itself these days isn't really that close to be a factor, but would be a convenient diversion.
Waddington - rather busy with resident aircraft and visitors, no spare hangarage. The proximity of the A15 and the road traffic issue which that would cause whenever the Team was departing or landing would be rather significant.
Wittering - full of plastic pigs flying UAS and AEF most of the time and much closer to the A1 than Leeming is to the A1M.
Of the 3, Leeming seems the obvious choice. Which no doubt means that Swanton Morley will actually be chosen!