NickB

One should distinguish between the various risks under the insurance policy.
For the majority of risks, what matters is the time elapsed between departure from the UK and return to the UK. This can be on different tickets, different modes of transport (train, boat, plane, etc...). Those two points will mark the beginning and end of the trip. If the duration of the trip is within the maximum specified in the contract of insurance for any single trip within the insurance period, then the individual is generally covered by the insurance whereas she is not covered if the duration of the trip exceeds the maximum period for a single trip.

Apart from that, the insurance policy will also cover specific risks associated with transportation itself, such as delayed departure, missed departure or missed connection, flight cancellation, etc.. The terms of these provisions in many travel insurance contracts sold in the UK typically restrict cover the the flight departing from te UK or returning to the UK, with any sub-journey nested into it not covered. For instance, if you book a return flight to AMS and a separate flight AMS-JFK-AMS, your travel insurance would cover you if you missed your UK-AMS flight for one of the reasons specified in the insurance terms (car breakdown, failure of public transport, typically) but you would not be covered if you missed the departure of your AMS-JFK (even though you would still be covered for other risks under the policy (such as, e.g., medical treatment and repatriation) wherever you are during your time away, including the Netherlands, the US or anywhere else within the territorial cover of the insurance.