APOD (France Fights On Mark II.) Goes in to some detail about Britain's economic policy. With France in thee war this gives the British Cabinet more breathing space for planning etc. While most of ths stuff deals with changes in equipment vs OTL there are some interesting titbits foreshadowing the UK and Commonwealth post war.
Since the ITTL there is no pressure to run British Industry in to the ground to prosecute the war (Britain can rely on the French as an ally, who in turn can rely on a generous supply of rented US equipment).
Instead, with the UK no longer getting LL aid, the government is forced to modernise its industry and import mainly industrial plant rather than finished goods to this end.
There have been references to modernisation of the shipbuilding industry thanks to the presence of French and Belgian labour shaming British yards into changing their wars toward the war's end. Also, innovative avenues of research that were cancelled IOTL are continued here, most notably automatic loading machines for coal fired engines, meaning that coal fired ships can stay competitive for longer (when combined with cheaper less labour intensive to build ships and low British wages relative to their US counterparts). References are also made to Britain having a thriving industrial and commercial electronic industry post war but details on the site are scarce.
Another thing to consider is that ITTL it is unlikely that Britain sells her assets in South America, and therefore has some financial reserves to facilitate a faster recovery.
In short, France Fights on has a Britain bled white rather than dry by the war, heavy industry undergoing modernisation and a much more stable finanicial base. Its Military is also much better equipped ( I won't go in to the details with ships, planes ,tanks etc, partly because they haven't been finalised, and partly because they're already apporaching Bible territory in terms of volume). There is also a secret agreement with Nationalist China allowing for periodical extensions of the lease on Hong Kong.
However, all of these changes do not a Superpower make, and I agree with previous posters that the UK is simply not large enough to be a superpower on its own. However, frequent references are made in the APOD discussions to much closer industrial and logistical cooperation with the Dominions, notably Canadian manufacture of aircraft such as the Hurricane, and the Reaper being outsourced to Australia. This lays the grounds for a Commonwealth that is much closer in practical terms, although I suspect that over time, the UK will lose its eventually role as leader in this organisation as it assumes and identity of its own and will be reduced to a (tenuous) First Among Equals.
Where this scenario falls down is that in this timeline, the Phrase “Superpower” will probably not exist. After all, the reason the USSR and USA were referred to as “The Superpowers” is because they were the only nations capable of exerting their influence on a Global scale. While they would be significantly more powerful economically and militarily than a unified Europe or Commonwealth, I doubt that the gap would be large enough for the idea of a superpower to take hold.
For Britain to be considered a superpower, it must be able to far outstrip all competitors exept for (at the most) a single antagonist, and I can't see that happening with a post 1939 POD.