Just so you know Screwfix do a 500w oil rad for £19.99.
I am quite lucky living in the far SW of England, our day time temps don't go below 10℃ very often, so my tent temps are fine during lights out. The lights keep things nice during lights on.
Lucky you. It's -20C outside today here in frosty northern Alberta. -40 with the wind chill the weatherman claims. Very little snow again so far this year. Can still see the wheat stubble in the fields beside us. I draw my grow room air from the greater area of the basement it's in so it's not so cold. Only 10.3C in the basement now.
Bought this at home Depot for $55 out the door. Has 3 settings and should work good. It's on a 20a GFCI dedicated just for my climate equipment. Already plugged in and testing in my tent in garage. Warming nicely.