Okay, new Loki plot as it stands now: Loki came to London to be treated by a specialist there. He's had the transplant and spent a long recovery period in the hospital. After being released he's still a bit weak mostly from lack of exercise. He doesn't expect Odin to want him back and needs to have occasional checkups at the hospital so decides to stay in England for now.

Drifting about aimlessly he comes through or near Lower Tadfield and is attracted by the concentration of mythological power. This doesn't have to have anything to do with Adam. (Maybe it was the reason the satanic order built its convent there in the first place.) The place is strengthening, maybe finally gives him enough power to shapeshift again, so he decides to stay for a few days.

War being there might be a reason to extend that stay. She's a reminder of his adventures with Odin and the only person around he knows. (Assuming that we don't manage to recruit an Odin or other AG character.)

What I'm still not sure about is the donor. I thought at first it could be any random unnamed person, but Loki will consider him his blood-brother and donating bone marrow isn't as simple as donating blood. Shadow was supposed to be a suitable donor, because of an unknown blood relationship between Odin and Loki that might have come to light through this ...
Maybe I should leave this question for after we have the full cast list.