Is a radiator valve a good substitute for capping of a pipe?

A radiator valve will isolate the water but will not always be reliable and it may not hold under pressure. Always cap the pipe or the valve outlet to ensure there won’t be any leaks

close both and cap them off,just shutting one side will not work,and trv's will open at 5c.

It’s is acceptable to use the radiator tap to cap off the flow when removing the radiator. I look at this as only temporary as the tap can leak or be turned on accidentally. If you are not Intending to replace the radiator, then a more permanent solution should be employed. A compression fitting is the Most secure option in my opinion.

It’s not a substitute for capping off a pipe, however if your dropping a rad for a short while, like to skim the wall or paint, as long as your there to keep an eye on it should be fine. Don’t leave overnight with heating running though without capping the outlets