Bizarre system

The £124 fare is an Anytime single between Plymouth and Lancaster, ie the most you would ever have to pay even if you just rolled up on the day.

The two fares you have split down to must be some combination of two Advance fares which restrict you to the specific train(s), and are non-refundable. So you enter the slightly grey area of 2 Advance tickets with different operators in the event of a missed connection.

The availability of through Advance tickets from Plymouth to Lancaster will also be pretty limited as it involves two different operators, Arriva CrossCountry and Virgin Trains.

There are a lot of journeys were an off-peak ticket isn't valid for the entire journey, but is valid for most of it.

Travel from Slough to Manchester via London is an example - Virgin will accept off-peak tickets after 9:15, but First Great Western won't accept them on the connecting train to London, because you're travelling during the peak. The National Rail website will tell you that a £200+ anytime ticket is needed, when in fact you need a £70 off-peak and a £5 single from Slough to London.

There are also a lot of anomolies regarding cheap day returns vs. more expensive off-peak returns, where if you're travelling back on the same day, two day returns for different parts of the journey work out cheaper.

Slough to Manchester via London is a slightly confusing one but should be able to be done on the £70.80 off peak return ticket. The ticket is priced by Virgin Trains, and is governed by the following relevant restriction,

Outward Travel
You may travel on any train that is scheduled to DEPART:
London Euston
Between 0905 & 1514 (inc.) and at or after 1832
(Between 0926 & 1500 (inc.) and at or after 1845 from January)

Return Travel
You may travel on any train that is scheduled to ARRIVE:
London Euston
At or after 1055
(Before 0720 or at or after 1130 from January)

Travel between Slough and Paddington should be unrestricted, thats certainly my reading of it.