Whether you need an 18″ or 20″ frame depends more on your inside leg length (and reach). I’m 6’2″ and run a 20″ 456Ti.
Thinking of ditching the full bouncer and going for one of these, £800 is a lot to spend on a hardtail frame (for me) but I like the look of it and have always fancied a frame in the magic metal.
And tbh £800 isn’t a lot for a frame, as no doubt you spent more than that on your FS (and shock).
I’ve had my Lynskey 456Ti nearly 5 years now – its done over 15k miles in that time, and a bit like Trigger’s Broom in that only the frame and the front mech are still original. Love it.