I think it would be better if, rather than making up GCSE grades (which wouldn't necessarily reflect what they actually know, just what they might have been expected to know in a normal year), they were to award everyone a "school-leaving certificate" instead. Keep English and maths, but for the rest, a certificate saying which subjects they studied, and perhaps comments from the school of the likely attainment had things been normal, and the student's commitment to study.
With hindsight, that would have been better this year, too. This way, you wouldn't have to worry about comparability with previous and future years. Everyone would know that GCSEs just didn't happen in 2020 and 2021, and that to apply any sort of requirement for GCSEs other than English and maths to those cohorts would be discriminatory.
You'd need some means of determining entry to post 16 courses, but I'm pretty sure sixth-forms and colleges could manage with teacher recommendations of suitability and/or asking to see some of their work. As it would mostly be on a local level, if a school was thought to be over-inflating their opinions, it would be easy enough for the sixth form/college to seek a meeting and ask to see the evidence.
Universities would have to rely more on predicted grades but also doable.
After that, who actually looks much at GCSE grades?