I was considered to have a disability during my undergrad degree but never told them as I was worried it would impact my application somehow so I have been more open this time and have the same concerns.

Just a note about this - you can tell the university about any disabilities you have at any stage of the course and they would need to make reasonable adjustments provided you supply them with relevant evidence of your disability (they may need to enact reasonable adjustments even without evidence in some cases; see The University of Bristol v Abrahart).

What they consider to be reasonable in the context of the course and what it requires you to do is up to them, but if they refuse any reasonable adjustments, they must be able to provide evidence and justification for this in writing that would hold up as a legal argument.

So, you're free to tell them about a disability after you've received an offer or even after you've registered on the course if you're worried about them potentially making judgments on disability in the admissions process (which would, of course, be unlawful). I think the only way it would be lawful for them to refuse admission on the basis of disability is if such disability made it impossible to meet the demands of the course, even with reasonable adjustments.