Thanks. Is it possible to just drain upstairs and not drain the boiler or is there no way to know when to stop draining unless you empty the whole lot?
As Duncan says, you can just drain the top floor. You will need to drain as if you were going to empty the whole system but check after a few minutes draining if the water is escaping from the bottom nut of the valves on the radiator. You can undo it a bit, maybe have to wiggle the pipe a bit to break the seal and see if water escapes. When sure the water is below that point you can disconnect the towel rail.
You'll want to connect your hose to the drain point and get it flowing, then go shut off the cold inlet to the feed and expansion tank, if your system is open vented. If it's a sealed system verify the filling loop is closed or disconnected if it has a hose linking it up. You don't want the system filling whilst you are trying to empty it!
Once you've got water flowing from your hose, you'll need to go open radiator bleed valves. Take a rag and a radiator bleed key and open the bleed valve on the highest rad on the system. If water issues from the bleed vent, quickly close it and try again in a minute or two. Use your rag to catch any water when doing this.
Drain down all the top floor rads in this manner including your towel radiator. Remember to check the Towel radiator valves until you are certain there is no water there. Close the drain off hose. Go and close all your bleed vents - Don't leave them open! Then you can remove the towel rad.
It is possible to just drain the radiator and not the other radiators on the floor but if one valve does not close properly and lets by, it will cause problems so I would suggest draining that floor. Others may disagree but that's my advice.
Either do the work you need to do straight away, or cap off the open pipe ends. You can usually find a compression stop end that will work with the radiator valve nut and olive still on the pipe.
Recommission in the reverse order. Add some inhibitor into the towel rail before you refill the system. If you don't know how just ask.