Honestly, this is the first time anyone's ever asked me to prove basic competence in English and Maths in the 45+ years since I left high school. Not sure if that's a "UK thing" or just a "UK Government thing".
It's the first time I've heard of someone who has higher degrees having to prove their GCSEs... and I'm British! Normally, you might be asked to write down on the application form whether you have GCSEs or not, but you wouldn't have to prove it!
In the UK, you have to pass GCSE Maths and English in order to be able to study for A Levels and/or get a degree... so once you have A levels, GCSEs don't really matter, and once you have a degree, neither GCSEs and A levels really matter anymore.
So, it's really only people who are looking for jobs straight out of high school, or who only have GCSEs and no other qualifications who might need to prove them.
However, I do state my GCSEs on my CV... something like:
9 GCSEs grades A*-C (including Maths and English)
