Only one thing to add: yes that Advance Single is inflexible, but not because of the 'Break of Journey' restriction - that is something else (which is only really relevant to flexible tickets and stopping off en-route).

It's inflexible because it's an Advance.

Your journey may not be the cheapest or quickest option, but it's perhaps the most relaxing. St Pancras is a very pleasant station, so no harm in hanging around there for a coffee and croissant if need be.