Hello,
Sorry for not spotting this comment for 5 months!
There are a number of "iD relation" issues, some of which you can see at https://github.com/openstreetmap/iD/issues?q=is%3Aissue+is%3Aopen+relation .
What happened here was basically due to https://github.com/openstreetmap/iD/issues/10184 .  When you remap something and you want to add it back to the relation, you need to choose which relation to add it to.  The fundamental problem is that iD does not show object IDs, so (here) you have to choose between the name "EuroVelo 2 - Capitals Route - part United Kingdom 5" and the name "EuroVelo 2 - Capitals Route - part United Kingdom".  The latter is "type=superroute", but iD doesn't show relation type either.
Also, when you hover over a way member of either relation iD will highlight all the same ways, because for superroutes it shows ways in constituent routes.
The (largely undocumented) answer is to find the relation number that you want by clicking on (in this case) https://www.openstreetmap.org/way/1272629446 and "view on OSM".  That tells you the relation number (in this case 5479817) and in iD when selecting a "parent relation" you can actually search (which isn't obvious) and can also search by number (even less so).
Cheers,
Andy