P7s vary from year to year but the heaviest ones are incredibly heavy, well over 6lbs.
Bikescene have Orange R8s in 17 inch for £270, that ticks most of the boxes and is as close as you’re going to get to your budget if you’re buying new.
TBH a lot of the cheaper/heavier steel frames end up being pretty solid lumps, just because of the sheer amount of metal in them. If you’re looking for the mythical springy, lively steel frame then you need to shop carefully. On One IMO don’t sell any IMO. (not to say they don’t sell good frames of course, but my Scandal and my C456 were more steel-ey than any Inbred or steel 456 I’ve ridden).
Material isn’t everything, there are lots of lumpy steel frames and lots of lively alu frames. And at your price point used carbon’s on the cards too. I’d say shop for the character you want rather than for a material.