I hope you won't think I'm advertising, but we might get some more cold weather.
I usually suggest oil-filled radiators with thermostats but no timers, because they give out a gentle sustained heat and I think they are safer. Tesco had a heap of them earlier but my local big store is sold out now and they have some convectors. When the weather turns cold high street shops sell out.
I happened to see that Halfords have some convectors at £19 I've had that brand before in an emergency. Sometimes they have special offers online.
Screwfix are the same price and Wickes are slightly cheaper
I keep a few in case my CH fails, but I got then in the summer sales.
It wouldn't have been the first place I'd look, but it might be useful. You can reserve online and pay when you collect from your local shop.