1. Very often the copper tails have either been bent to get them in / connected, or have to be bent to get them out. Although its soft copper I wouldn't want to use it if it has been bent and straightened a few times.
2. I'd suggest your best course is to:
2a. Turn off the hot and cold water before the isolation valves.
2b. Cut the pipes, either with a pipe slice (best) or a hacksaw and thoroughly deburr the edges inside and out. Cut them to a length appropriate to accommodate the flexis, the isolation valves, and flexi to iso valve adaptors.
2c. Get two adaptors.
Screwfix 2665R for 1/2" flexi ends, 6089R for 3/8" ends. Note the 1/2" and 3/8" are BSP threads, with internal diameters of 15.2 and 19.1 mm respectively. The ones you get depend on your flexis.
2d. If it were me I'd get two new, decent, full bore isolation valves. Something like
Screwfix 21860 for cold and 29086 for hot. You could reuse your existing ones, but I'd suggest new copper olives. Easiest way to get a few copper olives is to get a
Screwfix 95828.
2e. For each feed, attach the isolation valve to the cut copper pipe, insert the plain end of the adaptor into the other end of the valve and screw the flexi to the threaded end of the adaptor.