Could Britain do d day without Us support
All of below (aside from the aircraft production reference) is clearly speculation

. With that in mind:
As Shade mentioned, a lot of questions this question raises. Is Japan even in the war if the US isn't? If so, then the UK can free up a
lot of extra troops. However, without the US in the war, the UK would probably go through the Balkans - Churchill was as keen as mustard to go down this path, and without the US pushing for France, I'd bet good money that's where the UK would have started. I'd also think an invasion in the Balkans or Greece would start fairly well initially, as the defences would be less concentrated, although it would also be harder to pull off.
Another important question - is the USSR in the war? If not, the answer is probably going to be no, not in a million years. Commonwealth vs Germany/Italy is a long-term stalemate until someone gets the bomb (the UK had the lead in 1940/41, but without the US who knows who'd have come first).
If the UK and Commonwealth are the only significant players against Germany, Italy and Japan, then I'd say they should knock over Japan first (something they can achieve by themselves, although if they made as many mistakes as they did IRL, I'd wager it would take them until 1950 or so). If Japan is in the war, the Commonwealth need to knock them out one way or another to have the resources to land in Europe anywhere aside from Norway and Sicily.
Edit: Not sure what was going through my head when I wrote the above paragraph - if all Germany and Italy have to do is attack the UK, then it's highly unlikely they'd have the capacity to knock out Japan (and they'd probably be stretched to do anything but lose eventually, once the RAF and RN have been worn down enough - likely with Japan eating away at the periphery - alas poor Australia and New Zealand). Sorry about that, not sure what I was thinking - perhaps in my head I still had the USSR tying down Germany/Italy (although I clearly wrote it that they weren't in the picture). OK - back to original post now, I think the rest isn't quite as crazy

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Original post continues: If Japan is out of the picture, and the Soviets are in the game, then I'd say they could do it, but they would be better off 'nibbling away' at the periphery, taking Norway (still difficult, the Germans put 300,000 or so troops there later in the war in case of this), Crete, Greece and the like.
Not unless Germany had a rediculas failure in another theater and couldn't field an army.
The UK and Ger air force can go blow for blow being just about equal in strength each having some pros and cons
The UK out-produced Germany on planes from 1940 onwards, building larger planes (on-average) as well (and if the Soviets are in the war, the Luftwaffe is split to boot). It's all very "what if", but if we're talking 1944 historically but with the US watching on with popcorn, I'd argue the RAF would still be able to achieve air superiority over the landing area pretty comfortably.