Great Britain left the gold standard in 1931. At that point nothing other than ideology constrained the British government's ability to achieve full employment. The government certainly could have employed all of the previously-unemployed workers in war-related industries or in the armed forces. Whether that would have been a good use of British labor is a matter of priorities.
However, in answer to the follow-up question, expansion of the war industry and the military would not have been sustainable in the long run without price controls or increased taxes. Inevitably, the economy recovers from a downturn and the private sector begins hiring workers. If the government has pursued a policy of full employment, it now must make one of two choices:
(1) Encourage/allow workers it hired during the economic downturn to return to the private sector. There are many ways of doing so, and they're tangential to the original question. But the consequence will be a smaller military and war industry.
(2) Compete with the private sector for workers. During an economic downturn the private sector is dismissing workers, so by definition the demand for workers is less than supply and the government can hire workers without competing with the private sector. During a recovery, the private sector is hiring workers; if the government tries to retain workers hired during the downturn or hire more workers, it comes into conflict with the private sector. Since demand for labor exceeds supply, prices on labor will rise, creating inflationary pressure. The government will have to deal with this by either (1) imposing price controls on labor or goods and services; (2) increasing taxes to suppress private sector demand and create space for its own priorities; or (3) eating the consequences of uncontrolled inflation.
For example, Japan pursued aggressive fiscal policy during the interwar period. By 1936 the Japanese economy was operating at full capacity and experiencing inflationary pressure. But attempts to reduce military expenditures, which contributed the least to the real Japanese economy of all government spending, substantially contributed to a failed coup in 1936 which saw the assassination of several cabinet members. So the civilian government abandoned attempts to control the military or reduce its budget. The result, among others, was rampant inflation.
The United States controlled inflation during WW2 through a combination of rationing and price controls. Inflation in the US was not a real issue during the period between 1929-1940 because American fiscal policy was never aggressive enough to achieve full employment or even to reduce real unemployment into the single digits.