Would one radiator in this space be enough for 6.5 x 3.8m room?

Yeah that's more than enough I think. Make sure it's a double. What size rad have you worked it out to be? What's the width of it?

And how high is the ceiling?

6.5 metres x 3.8 metres is almost the same size as my Lounge and they put in two radiators (one either end of the room).
I also have a 5.5 KW wood burner in the middle of the room (on an external wall).
The back boiler also heats all my hot water in the winter.

I find that with the wood burner in use I can turn off both radiators and without the wood burner working the big radiator alone is good enough.

I think in your case you say its an awkward mission to put two radiators in so go for one but make it a big one.

On the coldest of cold nights you might decide to rethink things but I would follow Dans advice and go for one.

If you're speaking from experience on the same kinda room layout then maybe it needs to be a big double at least!

Don't be cold on my account

My lounge is 6.93 metres x 4.08 metres with a big double radiator one end and a small double the other.

There are a lot of things to think about but radiators heat the air and air moves about a room (especially if you have a wood burner).

If its a pain to put in two radiators just to have one radiator each end of the room then from my personal experience I did not need the small radiator and you might also get away with one big radiator.

Having said that you could start with two radiators and it is job done (provided you calculated correctly and the room is well insulated), or fit one and add a second later only if it is proven to need two radiators.

I can only share my experience of things in my own home but just because it works for me there is no guarantee

Roughly 2.5kw for a insulated cavity and one 1.4x1 m window

So being a condensing boiler your looking for an output at cataloge around 4.8kw so your looking at two 1400x600k2 or two 1100 x 600 k3