Which heating system to use in this kind of extension?

[vexation]

Your other option is to install a log burning stove - or perhaps even a hot/cold aircon unit.
If any of the windows are fixed, these could work:
https://www.theradiatorcentre.com/floor-standing

The prices on those trench rads are a bit steep! A log burner was one of my first thoughts as we already have an open fire in the living room although that's only to make things cozy. Not sure I could be arsed to maintain a fire as the only source of heat although they do put out an impressive amount. Also don't get me started on the price of logs! Already cut down and burnt most of the candidate trees in the garden

 

[motorbiking]

The log burner is just to assist on really cold days. Given the size of the room and the glass - your going to need at least 2 800w rads or a single 2KW - so your tower rad is going to be £500-800. Something like this. The low level rads in my above link start at about £400 - I do think you need to budget about £1K all in for this.

 

[ericmark]

Have you looked at fan assisted radiators, i.e. Myson, because it is forced air it can be mounted in kick spaces, and it is small in size.

 

[Lower]

My advice would be to invest in wet underfloor heating and do it properly on a seperate zone to the rest of the house.

The perception of warmth in the room is dramatic compared to conventional heating in an older property.

 

[vexation]

I'm finding some vertical rads that give us the required output for well under 1K but I'm still concerned that the area around the glass will get cold pooling being furthest away from the rad. motorbiking - those above ground rads would solve this but they all look ugly to my eye. I'd worry about tripping over them as well. ufh is back on unless something else pops up.

What we could really do with is something like a kick panel fan heater that goes down in a trench - low power like 50W per metre just to counter the losses around the perimeter.