It would be worth your while having a look at the Argos website as they do a very extensive range of different models. You are going to need a fairly high powered model say 3Kw or maybe two smaller radiators of 2Kw, as you have a fairly large room.
I take it the room room already exists. I would sugest that you take some time to see if you could improve the thermal insulation of the room. This is likely to make more comfortable in both summer/winter and also reduce the running costs of the radiators. Loth of 20 years ago often had minimal insulation and were notorous for being too hot or cold.
i was thinking of 2 smaller ones from the argos site theres a lot to choose from not sure which are the better ones,i was thinking of the delongi range
There is not much to chose between different models. The price tends to vary with the size i.e. 1Kw or 2Kw, controls such as timers and the design.
De Longi models are fine.
If you are going for two radiators, you might be interested this model from Screwfix, at a very good price
7Fin Oil-Filled Radiator - Screwfix.com, Where the Trade Buys
a very good price,do you know anybody has used one?