^^^^^^^^^^ given that the OP can do them for free, I see a certain benefit in collecting a few GCSE's, if only to demonstrate a rounded personality.
Passing a string of exams to get an ATPL, of itself, demonstrates "teachability" (an ability to learn, rather than simply memorise large lumps of arcane subject-theory.
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Unless you have exceptional abilities, I think you'd spend 2 years to amass the necessary passes at "O" level . I agree that school qualifications are likely to carry little weight where a mature job-applicant is concerned. Working -life is of much more relevance, but there again, the HR minions have boxes to tick.....have you considered writing to the head of recruitment (or possibly the Chief Pilot) of half a dozen airlines on your short-list, soliciting their views on what qualifications they would like to see in a candidate who has spent a few years in "the university of life" (it counts for a lot, in my book.
First get your Class1 medical- the whole outcome of your efforts depends on this.
Then, as suggested, look to GCSE Maths and English (you only have to look at many posts on this Forum to realise that many Pilots can't string a coherent sentence together consistently.
and don't get me started on Grammar, punctuation and the hideous unintentional Malapropisms caused by total mis-spelling of a word which sounds the same! Level 6 is, I believe, the standard required, which the OP posts indicate is easily achievable. -Maybe sit another language?Cantonese? Russian? Polish? Physics will undoubtedly be a big help to you as well.
Given that Airline Transport is international, the ability to speak a "less popular" language should be a real asset....if you've a linguistic flair, by all means do French, German, Spanish or Italian, as they are all pretty common, but still useful As @Flying Dutch will confirm, most of them seem to speak better English than the average Brit. Even the supermarket checkout operator transitioned seamlessly to perfect English when they realised I was a Furriner
Look at back-posts in the student forum....much advice, from buying secondhand course-books. (Airlaw, even the latest edition, is usually outdated before it's printed, but the exams are, also! "old" or" new" correct answer to a question gets a pass! you can show how good you are by giving both and annotating the fact you know that the law changed
You may want to save and get a share in a LAA Permit aircraft -or even buy outright....You can learn on it,certainly to PPL level and save a considerable amount. If you intend to do GCSE's and a Commercial licence, I'd guess you should aim for about 5 years (unless you win the Lottery and go full-time)
Get your medical, then set your stall out to achieve your goal. do not rush in with unbridled enthusiasm and then realise "I could have done it that way, a lot quicker and easier"
Good luck, go for it!
Note, I have no qualifications whatsoever, so the above could be worth what you paid for it. (in that case, the post will get "donked"