How do Silicon Valley technology firms view students studying Computer Science at UK universities in comparison to the ones studying in more well known US universities?

If you went to a top tier school MIT, Berkeley, Stanford, I am generally guaranteed a basic level of technical proficiency and it is  pretty safe to set up an onsite interview. If I haven't heard of your school, I'm going to spend a lot more time reviewing your resume, GitHub repo, and phone screening you before deciding whether I want to fly you in for an interview (potentially wasting time and money). I have met people who have supposed graduated with a degree in Computer Science who don't have things I expect a sophomore intern to know (e.g. what is a hash table, where would you use one, etc). On the flip side, I have met MANY people from school I've never heard of that are excellent engineers.

First, you'll find very few people who actually know much about UK universities other than Oxford/Cambridge. That doesn't matter anyway because the majority of Silicon Valley firms don't really care about where you graduated (except for some well funded startups that only look for talent at some specific universities). It's generally based on what you do and how you do things instead of where you come from. I think this mentality is one of the secret sauces that made the region become so innovative and productive - examples of successful college drop-outs are everywhere. This is considerably different than in the UK where the grade and the university you graduated from are one of the top aspects an employer looks at when making a hiring decision. Having said that, considering the immigration implications, US citizens have a little more advantage over non-US citizens. As an advice, bigger companies are the ones who would very happily sponsor your visa if you prove to be good in what you do, no matter what university you come from.

I doubt most Silicon Valley firms care what university you went to. You could easily be applying to work for a company founded by a college drop out. I suspect that if you come from the UK, you be perceived as a theoretician. If you want to compare to good Brits who did well for themselves here try working for Stephen Wolfram or Geoff Fox. BTW: they were physics majors/profs. Harvard's most successful "Valley" people were drop outs. Why not more favorable? Most American's don't study developments in other countries (Andy Tanenbaum found this out for himself, see the archives of the comp.os.research news group. Don't study. Code.