Starting your journey at an intermediate station constitutes a break of journey, which isn't permitted on Advance tickets.
Onboard the train, in practical terms there is barely any time between the two stations so it would not be likely to be obvious to the guard that you had boarded at Southsea and they are unlikely to care.
At Southsea station, your ticket may or may not work the barriers. If it doesn't, I'd say there is a fairly high chance that the staff will let you through without issue. However technically speaking they would be entitled to make you pay the difference to the cheapest valid walk-up fare, to be allowed through the barriers to board the train there.
Industry guidance does say not to charge such excesses unless there is evidence of intent to avoid a higher intermediate fare (unlikely here since the fares would both the same due to them being a group station). That guidance isn't something you can legally rely on if you're charged the excess, of course.
As long as you're willing to take the (small) risk of being charged the excess, then by all means go ahead and try to board at Southsea.