The restriction code for the £55 London-Bournemouth offpeak return is US, which contains no restrictions on the default position as regards break of journey. For an offpeak return, the default position is that you can break your journey (which includes starting your journey at a station after the origin station stated on your ticket, and also includes ending your journey at a station before the destination station stated on your ticket) in either direction.
It may be worth considering Advances, even as late as the day before travel, if by then you know the time you want to travel and stations you want to travel to/from.
