Your right to reject under CRA is 30 days.
So, you’re now into the realms of not fit for purpose / unsatisfactory quality / not as described. Can they be reasonably described as any of these things?
I’d expect a fitted screen protector to last longer than three months, so that would surely be unsatisfactory quality, and the onus is on the supplier to prove their wasn’t an inherent fault. Damaged in transit and then ignoring them for three months however, you’re on pretty loose ground. Playing devil’s advocate, if I were a seller I’d be tempted to file your explanation under “yeah, right.”
I guess realistically your options are 1) suck it up and don’t do it again, 2) appeal to the seller as a gesture of goodwill, or 3) lie. For the sake of ten quid I’d probably just bin them personally. It’s clearly a vital purchase if the tablet’s been in use for three months whilst they’ve been sat on a desk.