If you are based in London I would consider going to either Panavision or Movietech (in Pinewood), both camera rental houses are absolutely fantastic, I have worked with both and if you present the project in a nice and professional way they are more than happy to help.
It would be great if your cinematographer have worked with any of the rental camera places before so they will know him / her and that is always good.
All in all, 35mm camera rental fees are quite cheap nowadays so you might get a very good deal in any of the places above mentioned or on the ones you know.
Hiring equipment from abroad is an option if you get all or most of the stuff for free, then you will only have to pay UPS / FEDEX shipping charges (which are not bad at all) but you might want to get heavy stuff (let's say tripods / dollies / cranes / etc) in the country you're going to shoot your film.
Cinelab is a fantastic place to process film so that's a very good pick.
Have a good day.