Higher-powered radiators don't cost that much more than lower-powered ones and, because they've got variable heat settings, you don't need to use them at maximum power if you don't need to.
The only heating in my house is a Clarke 2.5kW oil-filled radiator and (having had loads of different heaters over the years) I can thoroughly recommend it to you. (You might well find that the 2kW model is actually fine for your needs as well but avoid the 1.5kW one, as it doesn't have a timer). They're available from Machine Mart, which has 65 branches across the country:
https://www.machinemart.co.uk/c/oil-filled-radiators/