Clearly, the answer is no - I don't see Britain alone being able to mount a D-Day without US support even when, as historically, the Russians were fighting the bulk of the German army. America's contribution was crucial in the air - gaining air superiority and doing significant damage through strategic bombing, at sea in securing supply routes and later providing troop transports, and of course on land in providing well over half the Western Allies army. Whether Russia could have defeated Germany without the US intervening (providing aid and keeping open the Western Front), I don't know - perhaps she could.

If the UK alone means minus US and Russia, I suspect the result would have been a reluctant peace around the end-1940 borders. Germany could over time have developed the air and sea power to cross the channel, but Hitler showed no real enthusiasm for such a costly programme. If Germany and Italy had pressured the British Empire enough - eg taking Egypt - I suspect Britain would have sued for peace and Hitler accepted.

I always liked Churchill's remark on hearing about Pearl Harbour and Germany's declaration of war on America:

"So, we've won after all."