I don't think my parents or husband will give me the choice not to!

In my opinion it is the supervisor that is the most important- or rather their importance/ reputation in the field- if they are well known and of a very good reputation, with an extensive publication record it is almost irrelevant which uni they are at. And bear in mind that some of the lower tier universities will have world class departments in certain subjects. Those working in the field will recognise which departments and/ or supervisors those are. Going for one of the top unis will not guarantee that your supervisor will be top in his field... And they may not have the expertise that you need for your particular project. It's only potentially useful to choose by overal university reputation if you are trying for a job in an unrelated area where the employer will only be able to recognise the overal prestige of the university.

Why just city hospitals? Your question sounds like you might benefit from seeing such implementation in a variety of settings. That doesn't have to make it very expensive necessarily though- for example there are hospitals based in smaller towns, with more rural communities, based within a reasonably short and cheap train ride from London.

Bear in mind that moving to another university won't necessarily take you too far from your fiancee- many university towns are within an hour of each other by train- commuting distance.

It depends- I did both my undergrad and my masters qualifications at the same university and recently I went back to do my PhD there too. I, too, worried about staying at the same institution, but in my case it really is one of the best places, if not the best place, in the country to study my subject, has the experts in the area of my PhD, and is in the city that I want to make the focus of my study (I'm an archaeologist)- it seemed daft to go anywhere else (especially as my husband and I own a flat here too). If the department is one of the top ones for your particular specialism then I think it's less of a problem to stay on- if it's not then you may have to explain your decision. Having said that, it is good to get different perspectives on your discipline. I try to go to plenty of conferences to get an idea of the range of viewpoints on my specialism that are out there, try to read fairly widely, and pick the brains of those who move to my institution from elsewhere.

I'm part-time and my fees are £1585 a year, so check the individual university before applying!
I work 3 days a week, and if you're organised it's perfectly possible!

And there's always the option of doing a PhD part-time, and having a part-time job to pay for it. Works for me!
Plus, just get your potential supervisor excited about your project (it's always good to meet them before applying/ or at least correspond) and your previous grades will mean a lot less.