Google has a number of doctors in their ranks. In any case it's not a requirement regardless, and equally a PhD programme doesn't give one jot what your A-levels were. They'll be looking at your final year dissertation/project, the quality of the research you did in that, and any other research experiences (summer projects, independent study projects in earlier years, any publications from any of these would be a huge benefit) as well as overall performance, particularly in theory and maths based modules. This is true anywhere, incidentally, although to varying extents.
It's also worth noting, in the UK PhD stipends are untaxed - so even if the stipend is "only" 18k, that's pretty much equivalent to a 24k grad job starting salary anyway, after taxes. Further, industrially sponsored doctorates and doctoral training programmes, which there are many (for example, all of the EngDs, and a number of other IDCs which lead to PhDs rather than EngDs) often have "enhanced" stipends, where the sponsoring company basically adds a few thousand a year into the mix. You will also get more if you study in London, no matter what. Beyond that, particularly in "traditional" PhD programmes there is ample opportunity to do additional work demonstrating labs, running tutorial/supervision sessions, marking work, etc, etc which also attracts additional pay. These sources are also untaxed to my knowledge. Being on a PhD is very different to being an undergraduate student - you won't be eating store brand instant ramen to make ends meet normally
There isn't really a need to contact google. A PhD will not inherently benefit or hinder an application. If the PhD is in a relevant area to where you'll be working, then certainly it'll be a good help. If not, it does demonstrate higher order research ability, which is very desirable, but if you're just working on an applications focused offshoot of google, it's probably not that important. It's more likely to benefit you than not, it's just the extent to which the is a benefit may vary - the "opportunity cost" of doing the PhD before entering the industry and earning may be larger for some roles than others. Overall a PhD will probably lead to higher overall salaries later down the line though.
HOWEVER - do not approach a PhD as a "tick box" activity to enhance a CV. This is absolutely not what a PhD is for, and if you do this and actually manage to get into a PhD programme, you will LOATHE your time on it. A PhD programme is something you should only go into if you're are fully prepared to be doing intensive academic research for a three year period. There's really no point in thinking about it while you're in school - remaining open to the possibility is good, but don't make any plans about it. If you do well in and enjoy your independent research in your degree further down the line (primarily but not exclusively your final year dissertation/thesis/project) then you should certainly consider applying to a PhD - if you're not hating it coming up to the winter break of third year it's probably a good idea to put together some applications, even if it's just to your home uni, to hedge your bets. However if the prospect of working on your dissertation/project just fills you with dread and/or boredom everyday in that first term, don't consider a PhD further at that point. Perhaps a masters, and then if your have a good experience in research in that revisit the idea of a PhD (sometimes a different superivsor can make a huge difference - this is something to be careful of when going into a doctoral programme as well) but don't plan to immediately go into one if you have a negative reaction to that kind of research. A PhD is essentially a 9-5 job of doing that, for a period of 3-4 years. it's a big commitment.
Also google don't care as much about where you went. Going to MIT or Cambridge isn't going to get you an automatic job offer from Google. They're just more likely to be of the intellectual calibre and technical ability to do well at google and in any assessments in the application process due to their own selection processes. A non-trivial number of google employees (as in, developers and engineers) have no degree at all. They care about what you do, not where you went. Someone whose daddy rang up Yale and got them an acceptance (not that this really happens any more, more back in the 80s) and coasted through in some generic major and decided they wanted to work at google so they can sit on a balance ball at work is probably not going to get very far in the process. Equally someone who went to an inner city school and worked very hard to excel above and beyond the standards they were raised in, despite the massive obstacles in their way, is much more likely to be emotionally and intellectually prepared for the rigours of life in such a role, and they're probably more likely to take that person - provided they demonstrate they can do the relevant parts of the role.
There isn't a magic formula to getting a job at google, just like there isn't a magic formula to getting in to Oxbridge. People like to think "oh well if you get 4A*s you must get in" or "oh if you go to CalTech you'll definitely be able to work at Google" but that's not true - these things are simply correlated
. Someone who got 4A*s is more likely to have the critical thinking and autodidactic skills Oxbridge want to see in students so they can make the most of their tutorial/supervision system and fast paced learning environment, and someone who thus goes to such a university and studies a relevant numerate subject can probably combine these suitably to be attractive to Google. But google isn't going to cross off anyone who didn't go there, and Oxbridge can and does take on students with lower grades, and reject those with those higher grades sometimes, if they invite them both to interview and one displayed a much higher ability to engage with their "style" - which rings true for Google as well. People want to discount the "soft" skills of critical thinking, thinking outside the box, approaching problems from unusual ways and thinking about the permutations of a problem, and boil the process down to a simple numerical tick box, but it doesn't work like that.