Am I allowed to take GCSE and A-levels in the same year?

I am in my late 20s and never did GCSE or anything that comes after. I want to take the exams around this time next year for entry during the 2021 cycle. I want to take the GCSE and A-level exams at the same time because otherwise it will delay me an extra year and I will have to apply in the 2022 cycle.

Having said all that, am I allowed to take the GCSE and A-levels in the same year and will universities accept that results from these exams from the same year?

i mean sure but that's a very heavy workload

While it is possible to take the exams in the same year, it isn’t very possible to do even one of GCSEs/A-levels in one year, never mind both

There might be exam clashes, but provided that isn't an issue and the exam centre you're sitting the exams in, there shouldn't be any issue in theory. I believe some exam boards might still have winter exam sessions for GCSEs; however A-levels can only be taken in the summer session for every board I'm aware of (May/June I think).

In terms of learning the content, if you're trying to learn the content for both several GCSEs and A-levels in a single year this is a very high amount of work. This is doubly true if your A-level subjects "follow on" from the GCSE content (e.g. Languages, Maths, Science) as you'll need to consider the sequencing of your study (although in theory the GCSE exams should be fairly straightforward compared to the A-level ones once you get to that point).

Since GCSEs are generally less important out of "core" subjects (English language, Maths, Science for some courses), and particularly for mature students they tend to only impose minimal requirements in that regard, I'd suggest only taking those you have to (such as Maths and Science) and focus on A-levels (or whatever) otherwise.

Many Access to HE courses do allow for students to sit the exams for GCSE English language and/or Maths (and sometimes will provide the teaching for these as well) so it's certainly possible to minimally take those subjects (said courses do normally warn it is a very high workload however) while learning other material. It may be worth considering an Access to HE course instead of A-levels if it's accepted by the universities and courses you wish to apply to (do check this well in advance) as these are designed to be studied in one year by mature students who often haven't had recent study, and so often have specific support for students in that situation.

I don't see any reason why the universities wouldn't accept it, it's all the same to them - the main thing for them is you'll have taken them recently so they'll know that's your current academic ability. Bear in mind though they may well not make any consideration for the fact you're taking all these subjects in a condensed time frame and may expect you perform at the same high level as someone who took them over a four year period (two for each GCSE and A-level study). Some universities and/or courses might be more flexible in that regard than others though.

Your best option would be take an access course in the appropriate subject such as humanities or science. These can be taken in your local FE college. Universities sometimes dont require GCSEs from mature students. Try checking out with the university whether they accept access courses and whether they forego GCSE's for mature students. Colleges sometimes require pre access courses before they allow you on the access course and it might be worth giving them a call to discuss this.