felt tired during race & a HM in 1:32 seven days before the marathon are probably linked. The old adage of take a days recovery for every mile raced means that you shouldn't have 'raced' a HM within two weeks (or 13 days) of the marathon.
the problem with flat marathons is the constant loading of your muscles, where the loading varies when going up & down hill. This means you wanted to do some long runs (in time as much as distance) to get your legs used to the consistent pattern of the flat surface.
Your London experience sounds similar to mine last year, two contributing factors were drinking too much water (causing stomach cramps) primarily due to being unaccustomed to the heat after training early on cold & damp winter mornings. Could this apply to you? Maybe look to
It doesn't sound like you followed the P&D plan very loosely, as you mention a few sub-MP paced runs on the flat, and your hard effort seven days before the race. You might get better results if you follow it more closely as it sounds like you made a few mistakes that their plans are designed to avoid.
... I suspect the "PR and corporate chums" don't need to bother with the uncertainty of the ballot.
For the OP, as people have said, normally the "official" opening time is 9am, but you may find you are able to access it before this time. I'd recommend trying as soon as you get up on the 22nd. I've previously managed to enter very early in the morning without any issue and then seen lots of people complaining of timeouts etc when trying from 9am onwards.
I'd look to get another race in sooner rather than later. Make sure you recover from the injury, and then book a 10k or similar in the next month or so.
This will prevent the DNF playing on your mind until your autumn marathon, as you'll have a positive feeling from completing a race. It will also give you an opportunity to channel all the hard-earned benefit from your marathon training into an alternative race.