Replacing Radiator Valve without draining system

hi everyone
Please see picture. i have plastic pipes for my central heating. i need to replace the faulty valve in the picture.
am however thinking of doing this without draining the whole system.
my question is, is there a tool that i can use to squeeze the plastic pipe just enough to stop the water flow, to swap the valve without damaging the pipe please?
house built in 1999 so plastic pipes are 20year old and am worried i might damage them using something like pliers with flat surface.

heating is gravity fed but rad is downstairs.

also any idea what size the valve is. i think its 10mm but will be nice to know am right ha

thank you

1. I wouldn't risk using a squeeze off tool on that pipe, or on any other barrier pipe for fear of damaging the barrier.
2. Looks like 10 or 12 mm pipe into a stem elbow. The other side of the elbow LOOKS like 15 mm to me. Measure pipe and stem to be sure.
3. I'd suggest you either freeze the pipe or drain down. There are ways of doing it by "bunging" the feed and expansion but unless you really know what you are doing, and there are no air leaks or faulty automatic air valves on the system, I wouldn't risk it.

Check the cistern for debris and bleed all the rads and cylinder coil.
Turn off the boiler and temporary plug the cistern outlet and cap the safety vent pipe.
Turn off the opposite end rad valve.
Swap the valve...if you do it in the right order you'll have minimal water loss.
Unplug and uncap etc.
Can't remember the last time I drained a system down..

Use a squeeze off tool. it'll be fine.

thanks All. can anyone recommend a not too expensive freeze tool. seems that will be the easiest if the freezing kit isnt too expensive. many thanks