Thank you very much for this explanation, I was confused by TFA (I skimmed through it, its my fault). Like I thought then, it's not a brexit thing at all.
It is a result of Brexit, as leading part (or all) of a Horizon project is a significant goal of many scientists' careers, and usually brings in much more funding than doing the smaller parts.
Britain lost that opportunity for several years, which will have caused people and groups to move to the EU.
The UK joining now means a group at Manchester University can lead the work on health outcomes, not just be a part of it under someone else.