Your GCSEs are not ideal, but they are practically at the bottom of the list of things that can make or break your application. I'm not aware of any set that has minimum standards for GCSEs, even though they do appear on the standard pupillage portal application. For all intents and purposes if you come out of your resits with AAA, your GCSEs will become all but irrelevant. A-Levels are more important. If you finish with ADD, or even something like ABC, that puts you in a more difficult position than if you come out with AAA, so that is something that you do need to work at. As for the resit point, the standard pupillage portal application doesn't distinguish between first results and resits as far as I am aware, and I can't recall ever seeing a non portal application form that requests original grades either, so for the most part you should be fine.
In terms of what you should be doing next, there are two elements to building an application when it comes to pupillage. The first is constructing a good paper application. Irrespective of how good you are as a candidate, it's not going to mean anything if you can't get interviews, and you get interviews on paper. So things such as A-Levels, your law degree (or non law degree and GDL), and the BPTC are boxes you have to tick, and you need to do them as well as you can. So AAA, a 2:1 and a Very Competent is fine and ticks those boxes for the most part, but AAA, a First and an Outstanding does it better. Equally you realistically need some other basic experience such as mini pupillages, mooting, debating and so on. Then you need additional elements to make yourself stand out, and I can't really tell you what that is because it varies from person to person. Once you've finished the academic stage, a good job with relevant experience can help you to stand out. Securing a particular prize or scholarship can help. But there are so many potential ways to do that. It's just about doing it as much as possible. So even if you have AAA, a First and an Outstanding, you should still be striving to make yourself stand out as much as you can. What achieves that year to year differs, so it's difficult for me to give precise examples. If I did, everyone would be doing it and it wouldn't make you stand out any more.
The second element is actually ensuring that you continue to progress so as to become a good candidate in person, and not just on paper. There is overlap here with the first element, because if you are a good candidate there are ways to demonstrate that on paper. For example, one of the main areas that good candidates stand out in is understanding how this job actually works in practice, and bridging that gap between the BPTC and real life practice. So few candidates relatively speaking understand that. If you do, it's something you will be able to express in person, but also to some degree on paper, and that can help your paper application stand out too. But having the best paper application in the world doesn't mean anything if you can't back it up in person, and you can't bluff a pupillage interview. You actually have to be good. So you should be aware of your personal development in terms of skills and knowledge as a distinct element from building that paper application. Essentially, make yourself the best candidate you can be. Again, there is no defined way of going about doing that, but it something that you need to be aware of and work at. So many candidates rely essentially on their belief that they're going to be a good barrister and struggle to actually back it up in practice. Don't be one of those people.