To paraphrase the Batteroo Terms and Conditions:
We do not give refunds just because Batteroo sleeves don't meet the claims we made for them
Apart from showing a contempt for their customers, they must surely be skating on thin ice from a consumer protection/ legal standpoint.
Since they don't provide a comprehensive list of products for which the sleeves fit and bring a performance benefit to, they are effectively transferring all the risk onto the purchaser. It is not good enough to say
Don’t worry, for every compartment where the Batteroo Boost does not fit, there will be others where it will fit fine.
because most purchasers will have a particular device in mind. I.e. Batteroo are selling something which they acknowledge is potentially useless and then pulling the shutters down when the customer returns.
I think it reasonable that retailers are not obliged to offer refunds if the customer purchases an incompatible product, as long as the retailer provided an accurate specification and did not misrepresent the product i.e. it was correctly sold but the purchaser made the mistake. However, with Batteroo, it is impossible for the purchaser to assess the compatibility of the sleeves until after purchase so, in their case, I find it hard to believe that they are not legally obliged to offer refunds.