Not all tickets can be booked at the moment, due to that day being too far in the future; not even Avanti allows you to select your return date.
Trainsplit would allow you to choose those dates, but returns an error message during the search.
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Also, since your Off-Peak ticket would be available on the day of travel at the same price, I would rather wait a bit so that Advance tickets hopefully become available for booking, as these are cheaper most of the time.
Furthermore, since you're travelling with a child, you might want to consider your train times, since 5 in the morning seems rather early.
Raileasy/Trainsplit, which our
forum's ticketing page is running on, is helpful in finding the cheapest combination of tickets available on your journey.
If it can't provide you with a split ticket, you can buy a regular ticket there with most railcard discounts (incl. the Family and Friends railcard) available and without a booking fee, plus you get to support this forum at the same time (if booking on the RailUKforums-branded page).
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However, it should be said that some splitting combinations just do not appear there, so it is not definitely the cheapest in all cases; there may be some people in this forum that know how to split tickets on this route to get a even lesser price.