can a towel rail be fitted in the bathroom without having to drain the water system.

It can be if you use a freezing kit... I would certainly recommend draining the system which doesnt take too long. nwtrades.com

I suggest to justify the cost,drain the sytstem, then when you fill up, put a new dose of inhibutor in. This will help your heating system last longer and be more efficent, saving you money in the long run. Regards Dean