You could replace the whole filling loop complete with valves or for a simpler solution you could just disconnect the braided link hose when not in use and cap off the valves with the supplied caps.
You also got to understand that we know what to do after years of training and experience and we come on these forums and give our time and knowledge free of charge. Good luck with the job and remember, its just physics. If you can remove the pressure and stop air getting into the system then...
Close flow and return valves on boiler.
Open bleed vent on radiator and catch water in cup or with towel.
When water stops close vent, put old towel under valve to be removed.
Loosen nuts on either side of valve.
Remove valve and replace with new.
Tighten nuts
Open flow and return valves
Check...
If I want my computer fixed I pay someone who knows what they are doing.
A pro could change your rad valve in 10 mins and spill no water. They would charge you for an hour but probably worth it.
On a Baxi 800 you should be able to safely close the flow and return valves. Then you do as Denso says, crack the joint, wait till the water stops and change the valve. I wouldn't even bother turning off other rads.
If you have a combi boiler, there will be an auto air vent within the boiler which will allow air into the system and water to then flow out of an open pipe if it is lower in the system. If you close the flow and return valves on the boiler you isolate both the auto air vent and the expansion...
Try Tap Magician. Looks like the cartridges you need are £10 each.
https://www.tapmagician.co.uk/NSManufacturersShoppingSelect.aspx?ENCB4AF132333&CDA&TT41
Is this the one you were trying to remove in your earlier post?
It doesn't seem to be visible when the tap is assembled so you may get away without having it at all. If you want to have a go at removing it, try wrapping a few turns of insulating tape round it and then hold with a pair of pump...
Looks like the longer one has been wound out too far and now won't go back as the hex shaped internals have rotated slightly. That's why it will only do one turn before it jams. You should be able to pop the circlip off the other end and tap out the internals. Clean it up, apply silicon grease...