Checking on Timetable World, the earliest in the Great British Bus Timetable was 0828 from Southampton. There was an earlier shortworking from Ower to Salisbury but not from Southampton; I guess it may have been a Romsey outstation journey?
If you could get yourself into Romsey by 0700 ish, you had a choice of the 34 or 36 to take you to Salisbury
Mentioned that in a post above; I am fairly sure that Ower journey actually started at Romsey. I do remember at the time a load of W+D double-deckers and minibuses would park up at Romsey overnight.
Getting to Salisbury before 0900 would have been critical for potential long journeys as that would have got you on the 0900 X4 to Bath.
So what's the furthest practical return trip in a day holding a morebus zone ABC day ticket, which covers all the way between Dorchester (on route 30), Southampton (on the Bluestar network) and Swindon (on route X5)?
Southampton to Swindon and return can be done easily, with an hour to spare, despite near-worst-possible connections at Salisbury:
SOU-SAL 09:10-10:32
SAL-SWI 11:25-13:22
SWI-SAL 13:40-15:38
SAL-SOU 16:45-18:00 (there is a 15:40 but discounting a 2-min connection as unrealistic)
With those timings you could easily start from and end at Lymington with a change at Totton, so Lymington-Swindon is definitely doable.
(If Bluestar still ran a two-hourly Southampton-Petersfield bus with the last bus back at 1840, as in the nineties, Petersfield-Swindon could have been possible, but sadly not these days...)
Or what about Swanage-Swindon, which is actually easier as the routes keep going later into the evening:
Swanage-Bournemouth 50 06:45 07:47
Bournemouth-Salisbury X3 08:05 09:35
Salisbury-Swindon X5 10:25 12:22
Swindon-Salisbury X5 12:40 14:38
Salisbury-Bournemouth X3 15:10 16:30
Bournemouth-Swanage 50 16:50 18:10
Dorchester seems to be difficult, even on the first bus you don't get into Poole until 12:15 (due to an hour wait at Wareham, you just miss the connection), then you've got to get to Bournemouth which would probably put you on the 13:05 to Salisbury and therby not get to Swindon until 17:22.
Can't see the old 184 Dorchester-Salisbury any more, or any equivalent, so it doesn't look like you can go that way anymore.
Nonetheless, looks like you could probably get back to Swanage as the last bus out of Poole isn't until 23:00. So perhaps the answer to the question is Dorchester-Weymouth-Wareham-Poole-Bournemouth-Salisbury-Swindon-Salisbury-Bournemouth-Poole-Swanage.
Presumably the ticket won't let you on the Salisbury-Bath, what used to be the X4, if that still exists (though I have a feeling that got split at Warminster a number of years ago)