CH radiator size

If the current room heat loss is 700Watts, and after it will be

The heat output of the radiator is also dependant of the feed and return temperature of the water flowing through the radiator. You may want to perhaps double the size if you are considering lower water temperatures in future either as part of a ASHP installation or making a gas boiler more efficient.

The B&Q WebSite is a good place to look online, they have loads shown (most of which are delivery only, ie not in a B&Q store) and the details given are pretty good, IIRC they all have their BTU output shown.

Vertical is good because it uses up less wall width wise, but they're less effective so need to be bigger, area wise. They're also good to lean against when you come in from the cold.

but much less likely to be connected TBOE ?

People often feel cold because their feet are cold when sitting down relaxing. Traditionally radiators were were installed under windows to counter the stream of cold air falling from the glass and circulating first at ground level. Removing a radiator co-located with the window, and increasing the glass size (even double glazed glass) may/will change the heating dynamics of the room. Perhaps consider going overboard with radiator sizing, but this is a bedroom so cold draughts at foot level may not be too much of a problem

The effective output of vertical radiators, and there less effective location, means the radiator needs greater capacity, than calculations might suggest.

Many, also require the flow and return pipes, to be fitted to the correct ports on the radiator.

also ... since the invention of TRVs the business of "going overboard" is not the problem that it used to be

I'm sure that's part of the reason, and the one I learnt at school, but another reason is that this usually puts the radiator out of the way. Plus double glazing must make this less of a problem.

Many insurance companies regard the tall radiators as “designer” models and often decline insurance on them. If you have space either side of the French doors you might be better off using two bog standard types.

On what basis? My insurer never even asks whether or not I have radiators, let alone what shape they are, I could heat the whole house using hundreds of IKEA tealights and they wouldn't know ...

Who "insures" their radiators? They're just part of the "fixtures and fittings".