I will be blunt with you,
The competition for any sort of sponsorship / mentorship is very tough, as I am sure you are aware. Without any grades you will find it very very hard to make it to stage 2 of any schemes, let alone the later stages. By law companies are not allowed to 'set' age limits / barriers on applications but yes you may be right in thinking they would prefer someone the age of 22 then a applicant who is aged 30+.
Next what concerned me is that, I am not exactly sure how you cant afford the time off to do GCSE's but are considering doing a degree? The time, money required would be much greater.
Possibly take a look at foundation degrees ? Might be one route but I am unsure of costs. Correspondance HND / A-Level learning?
I imagine there will be more replys to come, all the best.