Changing radiators - what is important to consider?

Most modern standard radiators are all pretty much the same. obviously you have to know what size you need but screwfix is a good place for you to purchase one, or any plumbers merchant. Or if you wanted to upgrade to a towel rail, all plumbers merchants will be able to help; Then You’ll just need to get a plumber in to change it for you.

The modern option is for a towel rail, they come in a range of colours and sizes. They provide heat and are ideal to warm your towels for when you get out of the bath/shower

You may wish to consider a duel fuel towel radiator that has electrical heating element for summer use, see link below: https://www.plumbworld.co.uk/heated-towel-rails-dual-fuel-31-0000

A new one will also rust be prepared for that nothing is sacred in a bathroom with steam and moisture

Yes they are mostly the same and standard ones can be bought from screwfix, however it’s worth bearing in mind that the whole system may need to be drain and inhibitor chemicals added, in some cases it might be worth replacing more than one if they are old as the corrosion from sludge takes place inside the radiator and can cause boiler issues

Most cheaper modern radiators are finished using a primer type finish (Screwfix), I'd purchase a decent make ( stelrads) for a much better finish in a room usually filled with steam/moisture, this will extend life and look better for a longer period of time, also if no extraction fan present in room I'd advise to install to combat damaging persistent moisture.