TL;DR - focus your training structure on the June Ironman; come what may with London. Allow the ankle to fully heal and don’t rush back too soon. Rainier’s plan is pretty good.
This is unfortunate, man, but sh&% happens sometimes. Don’t second guess yourself about your running and the injury; that’s counter productive. You can’t fix it now, just learn from it in the future.
Your ankle needs to heal fully before you start pressing training, even if that means you’re not going to PR both events. Up front, if I were your coach, even before your injury, I would’ve pushed you to choose one or the other as your real “PR” goal because training for a straight marathon and training for an Ironman are not the same. You will necessarily not be able to give your absolute best at both disciplines two months apart. A marathon will really tax your body and compromise your training for at least a week, probably longer, afterwards. So I think your race/training plan even before the injury is somewhat flawed, and that’s now compounded by the injury.
IMO, you WILL be able to have a really good marathon if you focus on your Ironman training, particularly with the injury now. You won’t be able to run for several weeks, maybe a month or more, if you allow your ankle to fully heal, which you should. So, I would focus my training structure around the Ironman in June, and do my best to run a strong marathon in London as my IM training allowed.
Let me caution you on coming back too early from an ankle injury: I suffered a number of sprained ankles from various sports over my teens and 20s. I very rarely took recovery from those sprains seriously, often just tightening up shoelaces or throwing a brace on and going back at it within a day or two, even sprains which had me on crutches for a few days, I’d hasten my comeback. It never seemed to matter too much until I dislocated my foot when I was 27, essentially destroying the soft tissue throughout the ankle joint. I was in a boot and unable to bear weight on my foot for two months, and full recovery of the leg strength took more than a year.
Take your ankle recovery seriously. Spend time in the water, even doing modified no-impact run training in the deep end of the pool. Listen to your body. Do not let the competitive spirit rush your comeback onto the bike and especially not into running simply because you have goal events. IMO, you need to reset your mindset for these events, and I would recommend structuring training around the IM and do your best under the circumstances at London. You have to think about the worst case: you rush back (particularly into road running) and re-injure the ankle in March… now what? You will be far better off allowing for full recovery now.
As the guys have said on the podcast, your “training” right now is to allow your ankle to heal. Do not compromise that by trying to come back too fast or maintain some kind of additional training load to compensate for loss of volume on the bike and road. It’ll be hard to do, but be smart about it and you’ll come out way better off on the back end.