Do you reccomend putting a stop or service valve between the pipework
and the TRV or lockshield?

I've never seen this. However, if I had such valves and wanted to remove a
radiator, I'd be very tempted to undo the compression side of the valves and
remove the radiator complete with them for mess free outside draining...

However, they would be ugly and I wouldn't bother unless under the
floorboards.

is it easier to drain the system to fix a valve than have
to move furniture and lift floorboards?

Draining and refilling is easy and gives you a good excuse to
flush/reinhibit the system. This can be a disadvantage if you've just put a
new load of inhibitor in.

Christian.