I have an exemption card, she said it wasn't a big deal just wear one. It's our policy.
YANBU, this is unlawful discrimination.
The Equality Act says that if you have a blanket rule that disadvantages some people because of a protected characteristic (e.g. disability) then this is indirect discrimination. Indirect discrimination is only lawful if it's a proportionate means to achieve a legitimate aim.
Nobody would dispute that preventing the spread of covid is a legitimate aim, however a blanket 'No Mask - No Entry - No Excuse' rule is not a proportionate means of achieving it. Masks just help a bit if enough of us wear them, that's all. The detriment to you - complete refusal of entry and service - is massively disproportionate. Your situation is even more disproportionate because it was a pub (one of the places where masks make the least amount of difference because everyone takes them off as soon as they sit down and has a good blather with their mates), because other customers were allowed in without masks, and because the staff were not wearing them effectively.
Providers of goods and services (e.g. pubs) are required to make reasonable adjustments for disabled people. The government has told businesses that some people are exempt from wearing a mask and sometimes this is because of a disability. They have deliberately referred to the EA in the mask legislation. Very obviously their intent is that the mask exemptions should be used by businesses as a reasonable adjustment for people with a disability.
For the avoidance of doubt, the government guidance for pubs says:
Some people don’t have to wear a face covering including for health, age or equality reasons. No one who is exempt from wearing a face covering should be denied entry if they are not wearing one.
www.gov.uk/guidance/working-safely-during-coronavirus-covid-19/restaurants-offering-takeaway-or-delivery#takeaways-6-1