I should think it goes a bit further back. It is also a question about capital and market.
The "modern" computer companies evolved from a lot of things. NCR was founded in 1884 and was focused on cash registers. The market and the amount of money created alloved it to grow into the electrical era and later into electronics and "real" computers. Not that many british companies had this ty pe of background (and vision for that matter).
IBM came out of different invention in the same time period (incorporated as IBM in 1911). In essence, it had a focus on banking very early on. Again the market in US was bigger than the British market and the amount of money generated got it into "real" computers.
AT&T coming from the "mini-Bell's" is another example. The innovations for getting PABX's and in the early days - switchboards - to go, was bleading edge (still is, btw).
I think it is the industrial scale that determined the US lead rather than separate innovations.
Prior to WWII, I don't think UK had any major focus on industried leading into electronics outside of some "funny" inventions and inventors.
I don't think they had the mney to fund anything either.
Post WWII, again, broke.
ICL was a range of mergers of different technologies, but again, I don't think the home market was big enough for ICL to grow cash and it lacked in international recognition.
(and on a different note: Silicon Valley in UK? next to M25? Reading? Oxford? the success of Silicon Valley is also the synergy among all the companies sitting there. I can't see a similar concentration of technology indiustries in the UK).
Ivan