I don't expect you to have too much trouble, there must be other taps in your flat off the supply, run those as suggested by others, clean any paint off first, then cut rapidly with a pipe slice that has a new wheel in it, take a nice new clean service valve with copper olives, make sure the olive is square inside by doing up the nut by hand until it is so but not tight, if you are really worried put a short piece of 15mm copper in the other end with a hose on leading to bath and turn service valve on as someone else suggested, but I probably wouldn't bother if I were doing it, get a good strong grip of your pipe slice and pull it off like a man, bend the upper pipe away, slide your service valve on, holding it downward with one hand get your grips onto body of service valve with other hand, holding in downward direction free up first hand to take your wrench and tighten up olive, you're safe.