As long as they avoid things like racial discrimination any shop is entitled to charge what they like, to whom they like (or dislike), when they like.
For example, Alf could walk into the shop and ask to buy that umbrella. The shop could say "That will be �25, please".
Bert, who's standing next to him, could ask for a similar brolly. The shop could say "That will be a hundred million pounds, please".
Charlie could come into the shop and try to buy an umbrella. The shop could simply say "Tough luck mate. We don't sell to people wearing cardigans".
Dave could be the next to try, with the response "We'll sell you the umbrella for just 1p, but only on condition that you hand over the money while standing on your head, stark naked, singing the Latvian national anthem backwards".
It's all perfectly legal and long may it remain so!
You were offered a contract to purchase the umbrella for �25 and you freely entered into that contract. Your contract is completely separate to that of anyone else (such as a person who paid �20 for the umbrella) and you've absolutely no right to a refund.
Chris