Simba sell off their returned mattresses on eBay fully professionally cleaned and refurbished.
You can get a £700 mattress at times for £150.
Bought one for the spare room and it’s actually nicer than the main mattress we have. Depends how fussy you are sleeping on something used.
There’s also a guy on YouTube that reviews mattresses and gives you an idea what’s best for you dependant on weight and way you sleep