>>> PAT tests cost £25 to £35 each
Only if you allow someone to rip you off! An auctioneer I know pays someone to come in once per week to PAT check the stuff in his sales and pays £2 per item. Charity shops that do sell electrical goods often have one person who is trained to carry out PAT tests, who periodically visits each of their shops; others have local electricians who do the task on a purely voluntary basis.
There is no specific law (or regulation) which requires that PAT tests are carried out on electrical items offered for sale. However there are general requirements which businesses (including charity shops) need to comply with under (for example) the Health and Safety at Work Act 1974 and the Electricity at Work Regulations 1989. In order to meet with such general requirements, it's considered best practice to carry out PAT tests (and, indeed, might well be a requirement of the those providing insurance cover to the businesses).
I'm certainly surprised to read about a charity shop selling untested electrical goods but they're not committing any offence by doing so. (There's actually an independent charity shop in Stowmarket which operates to support children's charities but which isn't run by those charities. They happily offer untested electrical goods for sale with no cautionary notices at all!).