Britain, empire, and a long war of attrition

Could Britain defeat Germany without the United States or Soviet Union doing the decisive work?

Yes, over a long war, by combining industrial and naval superiority with the resources of the empire and sustained economic pressure.

Britain was not a small isolated state waiting for a larger ally to win the war. It had a substantial industrial base, heavy bombers, a mechanized army, and a fleet far larger than Germany's. A land campaign without an ally would be difficult, but the traditional British method combined naval and economic pressure with financing and supplying continental armies. Canada, Australia, South Africa, India, and other imperial resources expanded the available personnel, production, and raw materials. A Britain-Germany war would be long, but German economic exhaustion still made eventual British victory plausible.