Did the British have any knowledge of the development of the A-bomb? What about the Soviets? Where there any plans in the event the war ended before the bomb could be used?
Both British and Soviets were aware of Manhattan project before it officially begun in 1942.
British were involved from the start, since they shared their own nuclear research with US, while Soviets were kept up to date on the status of Manhattan project since British Intelligence was overrun by Soviet spies (Cambridge Five, for example): first confirmed reports Soviets got about nuclear weapons could be traced to September-November 1941 (London Uranium committee meetings).
Moreover, if G.R.Jordan is to be believed (his allegations are controversial, to put it mildly; but "From Major Jordan's Diaries" itself is considered to be quite authoritative source on non-nuclear Lend Lease shipments) Soviets were getting nuclear materials directly from the US by 1942 under the Lend Lease program (at least until 1944). I.e. Manhattan project was literally supplying Kremlin with samples. Unfortunately, all Lend Lease documents are still not accessible in their full glory, while official investigations (HUAC, FBI and others) on the matter were inconclusive.