Given the choice of cheap(ish) books, I'd almost always be more likely to pick up the paperback for space-on-shelf/weight-in-hand reasons, and I'm sure I'm not alone. So pricing the hardback significantly lower may well be their best bet at shifting it. Quite possibly other types of hardback, eg cookery books(?) may sell better as harder-wearing and so be priced more? Seems sensible market-forces to me, just because hardbacks are *originally* priced higher doesn't mean 2nd-hand hardbacks are easy to shift without lower prices.

(Alternatively, their pricing may be entirely random :-) But many charity shops now do have a dedicated book-sorting/pricing person - my sister-in-law volunteers as one - so there probably is some policy.)

I'm sure you're thought of this, and I don't know Oxon's policies, but some libraries have reduced/waived fees for certain things which may include reservations etc for disabled users. Worth asking if they do and whether you'd qualify? (I don't know how 'officially' disabled one has to be, and whether you currently count as that - I currently do not, beaurocracy being what it is, but some things let you self-define to greater or lesser degree.)