A corollary question would be "how important was the Western Front/Strategic Air Command/Northern Lend-Lease routes to Germany's defeat?"

The primary contribution of the UK, in my opinion, was their actual landmass on the eastern side of the Atlantic that made those other things possible. (Not intending to minimize their contribution in terms of manpower, material, or morale).

Are you including the Commonwealth and the Raj as part of the UK for this question, or are you just refering to the UK as we have it today?