Employment prospects after, won't make much difference as long as you do a placement year then that will give you a better chance of securing employment.
Tbh for mech Eng employers aren't really that bothered where u go, what they will look for is someone who is already good with the SOFTWARE e.g. matlab autocad etc as you will be using them alot.
Coventry uni has nowhere near the prestige of those unis you applied to but it has direct links to F1, Red Bull, Jaguar Land Rover etc. so for Mech eng I'd say the links matter more than prestige.
Ucl being in central london puts you closer for doing interviews etc but since most applications are done online, it won't make much difference in term of getting the interviews.
Have a look at the careers fairs and see whose there,
Durham was awarded Uni status at roughly same time as UCL, but has a history which predates it so is arguably more "elitist" and is seen as an oxbridge alternative. UCL has the London brand name. Tony blairs son went to bristol.
3 very different locations, so for me personally I'd pick Durham or Bristol as I don't like London. You might love london though and its international feel