Turing and his Bombes for the deciphering of Enigma are certainly the most famous but to my mind the real missed opportunity of British computing was
Colossus, the first electronic digital computer that was programmable, to a certain degree, that was used for the breaking of the Lorenz cipher machine traffic which carried German strategic-level communications. Rather than building on it the government, for seemingly logical reasons at the time, classified things so heavily the true story didn't come out for thirty years or more and most of the records were ordered destroyed. Tommy Flowers went back to the Post Office Research Station where due to the secrecy he was denied most of the credit he deserved, even there though he wasn't able to get his designs for electronic telephone exchanges advanced. With all of this the UK was still a competitor to any other country in the field - the Manchester Mk 1 being the first transistorised computer, the Ferranti 1 developed on from it being the first commercial computer, the Lyons Electronic Office being the first real business computer, the Ferranti Atlas as the first computer with virtual memory and other examples. Where the Americans had the advantage was the government being willing and able to spend on early powerful computers for military and civil research plus having a larger potential market. The British also made a number of strategic errors such as their perennial one of having too many small companies competing against each other, and when they finally consolidated in a series of amalgamations tried to compete with the giant in the market IBM with their own British-designed technology when instead they should have been producing IBM-compatible machines or going in the opposite direction try to identify technological alternatives to IBM's stranglehold on the mainframe market.
If you want the UK to do better then you need them to be slightly more selective and not as heavy handed on what parts of the Colossus developments they keep secret. I read Paul Gannon's eponymously titled book
Colossus a little while back and he argues that whilst it might not have affected the design of the early post-war British computers what knowledge of the achievements would have provided would have been the major psychological boost given. Having a report or two written about the de-classified parts as happened in the US would have allowed better dissemination of information and for individual scientists or groups to talk to each other and swap ideas. The other big thing you need to try and do is accelerate the mergers of the various commercial concerns sooner so that the remaining companies are larger and financially more stable, my personal opinion is that two or possibly three companies would have been the best number since it balances economy of size with still retaining competition.
