well building regs in the UK state that the floor should be capable of 40lb per SQ FT. It should also have a safety margin of 2/3 times this; 120 lb per Sq FT. this means that floor joists of new buildings can be 40mm X 70mm as oposed to the previous standard of 50mmX 80/90mm. this also means that new builds can use metal cups as opposed to being jointed into the masonary. I really cannot apply this to your location but from a safety point of view, if you are afraid you will exceed 100lb per sq ft it may be worth getting a structural engineer in to assertain whether or not the floor is suitable. A 55g is no where near this and therfore should be fine, be it in the UK or elsewhere.
I know a lot of people have and do install large tanks upstairs, however there is a certain point where you must be safe as opposed to sorry. In my situation, the tank is directly above a resuraunt and if it went through the floor it could be.... messy.
There are more complications as i'm sure lots of people are aware but the above is to simplify. i don't pretend to know everything...
A 55 gallon is fine. I'm pretty sure Twk1 is right, Gr8 has a 240 upstairs. I've got a 300 gallon tank on the second floor, but it only has about 200 gallons in it (Snappy doesn't like it too deep). I also had a 90 upstairs in my house for years, as well as a 110.
Should be no problem. I would use some 2"X6" spreader boards to spread out the floor bearing wt just to be sure. I would also find out which way the joists run and set up the tank running across the joists.