If universities did not think an A was similar to an 8, they wouldn’t equate it as such.*
This isn't necessarily true. The English GCSE grading system doesn't align with the Welsh & Northern Irish ones above the 6/7 boundary (allowing for Northern Ireland introducing C to be equivalent to 5) but universities don't have any mechanism for judging whether an 8 is equivalent to A or A without requesting raw marks and assessing these against published grade boundaries, which we don't have the resources to do. Treating 8 as directly equivalent to either A or A* is inevitably going to be unfair on some students but there isn't really anything universities can do about it.
I don't think an 8 is directly equivalent to an A, and wouldn't be comfortable distinguishing between As and As at GCSE for this reason. Other medical schools depend more on GCSEs for selection, so they are forced into stating an equivalence even if they don't believe it's fair. I don't have any power as an admissions tutor to introduce any method of discriminating between 8s and 9s that would be fair to students from Wales or taking CCEA qualifications in Northern Ireland.
The problem is that English, Welsh and Northern Irish GCSEs are all officially treated as the same qualification, which they aren't. The same goes for A-levels. There is no way that a grade in a Welsh A-level is directly equivalent to the same grade in an English A-level. But, then again, it's difficult to be entirely confident that a grade in an Edexcel A-level is directly equivalent to the same grade in an AQA or OCR A-level. Ofqual at least accredits all of the English exam boards' qualifications, but the alignment of standards between England, Wales & Northern Ireland is a bit less reliable.
The school qualifications system (outside Scotland, which I confess I don't know much about) is an almighty mess, and gets worse every time governments touch it. But, of course, degree classes also aren't equivalent between universities, so wading through this mess sets up students for the complications they'll face later.