Your academics are the most important part of any paper application. If you land a few interviews, no doubt your strong character, as reflected by your dedication to EC work, will shine through.
In my opinion, your degree class is the most important aspect of your academic credentials followed by;
2) Awards/prizes/Scholarships
3) University attended
4) GDL result
5) BPTC result
6) A-levels
7) Masters.
You probably won’t even be asked for your GCSEs grades for non-pupillage portal criminal/common law sets so forget about your straight As impressing the pupillage sifters. Nice to tell your friends and provide you with some comfort, but not much else. Sorry (exception would be commercial sets)
If you get a first, great. You should aim to obtain a Inns of Court Scholarship and win debating/mooting awards while on GDL. You won’t feel great if you have nothing to write under this standard column on most application forms. I wouldn’t fret too much about university attended. In 2009/10, 4 people obtained pupillage who had graduated from the OU. To put this into context, only 12 people obtained pupillage who graduated from Durham University that year, and Durham accounted for the 5th highest number of graduates who obtained pupillage – behind Oxbridge/UCL/Sheffield. In my anecdotal experience, a 1st from OU is better than a 2:1 from anywhere else (apart from Oxbridge)
Others take a different view, but unless you are doing a BCL , my opinion is that doing an MA betrays a fundamental lack of imagination (unless you LOVE the subject and got a 1st on UG course). It will look as though you are masking poor grades elsewhere. (which you are). My advice would be to nail the GDL. This is such a difficult course and if you get a distinction – a fantastic result. I would submit pupillage applications while completing the GDL and see how things went. If you get more than 2/3 interviews then you know that it not your paper application (i.e. academics) which are holding you back so you should feel reasonably confident that you should undertake the BPTC. If you get no interviews then I would seriously consider taking 1/2 years off before you do the BPTC. I would re-do my A-levels while doing a Masters. Throw in some innovative EC work (like going abroad) and you would be in a much stronger position than forking out 17k in advance. I wouldn’t worry about age. Many pupils are in their late 20s/early 30s anyway. In my experience, only the commercial sets pick up candidates who are 21/22 as they are most concerned with cherry-picking the best talent.
You must be aware that the latest round of legal aid cuts come into force in April. As someone in the know, I can assure you that criminal/family bar will look very different in 4-5 years. Chambers will merge/close. There is no getting around the fact that there is an oversupply of legal aid barristers. Given this fact, there will be even less pupillages going in a couple of years and so you need to be the best possible candidate (i.e. great academics – hence re-do A-levels) and you will know which sets are likely to survive. (you might not think that is important – but you want a career, not just pupillage at a set with no future) I have no doubt that some unfortunate souls will lose their pupillage before they start because their accepted chambers will go bust. Things are that bad.
However, if I was you…I would not even do a BPTC. If you intended to practice criminal/family law, I would complete the LPC and get my higher rights. Why are you fighting so hard to join a profession that will be dead in a few years - (you wouldn't have wanted to be a qualified miner looking for your first job in 1985 would you? As a solicitor, you still get to do the advocacy, wear a wig and defend clients. You also have the added bonus of being able to pay for a mortgage (just) and weekly shopping.
I took out a professional loan in 2009 and paying it off as we speak. I have to pay back approx £400 per month rain, sleet or snow. It is a huge burden even with a relatively well paying job and pupillage kicking off in October. It is not a decision to take lightly. Don't load yourself up with debt unless you have control of the "controlables" and given yourself the best possible chance to succeed.
Best of luck!