I've got insurance with Essential and they mention that starting in the UK thing.
I take that to mean that as long as I leave the UK and return sometime in the maximum 45 day period, anything in-between is classed part of that "up to 45 days" trip.
Most people don't do straight A to B travel with a 100% fixed itinerary so unless you're planning on visiting somewhere with a higher associated risk i.e. Chernobyl, Rio's Favelas etc., side trips should be fine.