what about using one of these push fit isolation valves?
http://www.screwfix.com/prods/25911/...15mm-Pack-of-5

- Clean the feed pipe up before cutting it.
- Set the valve to be closed
- cut the pipe and put your finger over the cut end to stop the water
coming out
- quickly push the fitting on.

You now have an isolation valve ready for next time and can change the
taps in your own good time...

Provided you are fairly quick, you should only lose about a cup or so
of water...