I'm afraid I can't give specific advise, but I'll say hello
I suspect that you're right about London and GNR. On the other hand they're difficult races to get any entry as they're so popular, so you should stand more of a chance of getting into the youth races.
Regarding your training, the general rule is do not increase anything by more than 10% - for instance don't ever increase your distance by more than 10% (learn from my mistakes

) The main advice is simply listen to your body, if it's hurting and telling you to rest, then rest!
Good luck with the training!