I think either itinerary would be fun for a 7 year old. We took our 3 kids (ages 5-8 at the time) on on the
DCL British Isles Cruise and we had a fantastic time exploring London (and Leeds and Dover Castles) for a week prior the cruise.
Full disclosure---our kids are more interested in history than most kids their ages, but this is just because they have been exposed to a lot of history bc I'm a history lover. We cruised with friends whose 8 year old daughter enjoyed herself quite a bit--they just made sure to read some interesting children's history books with her beforehand. I think exposing a child to the history pre-trip (books, playmobil/toys, audiobooks of history or historical fiction, videos, etc.) is key to maximizing their enjoyment--although I think most kids could enjoy exploring castles like the Tower of London or Dover Castle regardless.
We were physically active doing a lot of walking and exploring (with beautiful scenery), but it wasn't a nature adventure type of trip. I think the Norwegian Fjords would give a mix of nature hiking/rib boats and Viking history (or you could do nothing but nature if you preferred), so if your child isn't interested in history yet, the fjords might be an easier fit for now.
Either trip sounds spectacular to me. We loved the British Isles and want to eventually go on a Norwegian Fjords cruise with our kids. I'd rather explore Iceland on a land trip to see it more thoroughly, so I'd personally lean against doing it on a cruise, but no doubt it would be fun too.