I'd go for an oil-filled radiator. The Clarke brand, which is sold by Machine Mart, is excellent. I use one, next to my desk, to heat my living room. (Even though you might be able to get away with using one of the cheaper, lower-powered models in a very small box room, I still suggest considering a 2kW or 2.5kW model. Multi-finned radiators are more efficient and they'll give you an extra boost of heat on a really cold night but you can still use them on lower settings for normal use):
https://www.machinemart.co.uk/c/oil-filled-radiators/
(The nearest Machine Mart stores to you are 26 miles away in Worcester or 29 miles away in Gloucester but they'll deliver online orders to your home for just £4.99).