I think the Giroflex G64 is the best chair I've ever used. It's very adjustable, the base tilts, the back tilts. The padding is not too squishy. It's also the right size to provide support, i.e. not too big so that the back rest is too far back. I would try one, but on your budget you'll have to buy second hand from an office clearance place as I think they are £400+ new.
I currently have an Orangebox X10 as that's all my local office clearance place had (I wanted a Giroflex G64). The X10 is a reasonable quality chair, the padding is not too squishy, but the base doesn't tilt and the back is too far back for my liking.
Rgds