gonna add in my input, hopefully it helps! one of my mates is currently in her third year of physics PhD at Cambridge
she said the admission for a PhD there is highly competitive baring in mind she did her BSc and MSc at Cambridge; as far as she can remember the admission data for a PhD in her dept was 28 out of 380 applications
-your supervisors push and motivate you to finish your PhD in 3yrs max upto 3.5yrs
-working as a TA/RA is highly unlikely but if you want to make extra money its possible (limited to 6hr per week)
-to enter into a PhD with a bachelor's is very very very rare but an MSc or MPhill would make it easier
-there is a healthy balance btw your academic and social life
the major difference btw a PhD from US and UK is that, US universities require pre-requisites and subjects that aren't of any relation to the degree : no-offence intended mate (same reason bachelors is a year longer compared to UK) and up to a years worth of research training thus takes btw 4-7yrs to complete; but UK universities are very much on point only require you to attend few lectures to help your research (mostly in the first 3-4 months of your PhD) and they expect students to have enough research experience from their MSc (here MSc is tailored to be more research/experimental than theory) thus takes <4yrs to complete
-UK students are guaranteed funding because ESRC prioritizes British citizens over any other nationals; hence funding for other nationals isn't guaranteed
-the stipend covers tuition fee and is more than enough to live on, typically ranging btw £13000 to £19000 depending on the university and location
i suggest you make sure that the university you intend to attend in UK guarantees funding otherwise living costs in Cambridge or Oxford are almost same as London (not central London but more likely towards greater London)
here's a link for typical annual costs for attending Cambridge
http://www.admin.cam.ac.uk/students/studentregistry/fees/costs/coursecost/costs2013v9.pdf