This article says that 70% of them just didn't bring any ID with them. That's on them.

It also says nothing about the remaining 30% except that they "brough a type that was not accepted". We don't actually know if they even had the right to vote.

https://www.reuters.com/world/uk/about-14000-people-england-...

Those who did have the right to vote could have gotten a free Voter Authority Certificate before the election:

https://www.gov.uk/apply-for-photo-id-voter-authority-certif...