I think if I wanted to buy a whole rigid, I’d get one of the many hardtails with a cheap rubbish fork, and an Exotic carbon fork, and introduce them.
This route +1.
And single speed it whilst you’re at it. The two complement each other brilliantly, the rigid bike lets you smash up hills without worrying about bobbing suspension, and singlespeed encourages you to do that. and with a few careful choices it’s possible to build cheap bikes that can easily go 12+ months with nothing more than brake pads, or sometimes even without wearing those if you compromise and go for tiny rotors (small rotors get hot quickly, so stay bedded in even in the middle ow winter when others pads are being worn away because the surface never gets a chance to harden).
Secondhand, stuff like Singular Swifts, Salsa El Mariachi, or at the cheaper end Trek Marlin SS, Cannondale Trail SS. Stooge if they fit you and you can find one.
Or buy new and custom build it, with a £900 budget, and no need for a groupset or suspension fork you’re well into the bling end of rigid bikes! But as noted above, you can take any bike, add a £100 carbon fork, strip the gears off it, add a tensioner and make it singlespeed. Once I’ve got a new job my plan is to get a carbon 29er and singlespeed it, there’s something really fun about riding a bike that weighs sod all!