I can give more details if needed
There is no rules that says the outbound and/or inbound travel must to from/to a specific place. You don't even need to use an outbound/inbound travel day.
If I'm not mistaken, this only refers to the country, and not to a city or municipality where you live.
@rvdborgt , will be able to say this with certainty.
@AnnaB is right, there are no rules as to what your departure or destination station should be when using the inbound/outbound journeys. You can choose whatever you want.
In GB the only-and as such fairly reasonable rule is- that the route taken should be a reasonable straightforward direct. As such this should be in all countries, but to sky-high fares in GB and the myriad of train companies, they seem to take it a little more strict as elsewhere. (I dont live there, but this from well-informed staff fora for UK-travels)
Thus in your case f.e. not go to MAN (or Leeds or York) first and then same day back to homey Southpt. You will have to find a ticket yourself to do later Man-Southpt.