ASK means Available Seat Kilometeres - sorry you're not familiar with this. It may not be a common term in the railway industry. Perhaps it should be, as apparently it makes a big difference...
As for an example, take my journey I am making today. FGW wants £38.60 for an off peak return. I have a "choice" of a 1pm return journey, or a 7pm return journey. Hardly that flexible, is it? It is so inflexible I went ahead with advance tickets (combination of Chiltern, London Midland and Southern). £26.00 return. Takes as much time as going directly on FGW, unbelievably. Relative saving of about a third, with hardly any less flexibility.
No it's not a common railway term, as it's airline jargon, I now discover.
Your journey would be what, exactly, and at what times? How did you book it? On the day or well in advance? A single transaction or separate ones? It's a bit hard to know what you're talking about if you don't give basic information like that. If it was multiple transactions, that would be well outside the realm of your "average person".
A Cotswold Line off-peak return - if you actually mean an off-peak return as opposed to an off-peak day return, which is a different ticket - is simply not time-restricted for travel back from London on FGW in the way you are suggesting.
And miss a train along with way with your advances and wave goodbye to your 'saving'.
I did not say it was impossible to save money compared with FGW fares - but for your next trick, perhaps you can demonstrate a way, travelling to London in the peak, leaving Worcester at about 7.30am and reaching London before 10am, either using rail via Birmingham, or driving to Warwick to use Chiltern, and returning from London at about 17.15, to beat the FGW off-peak return of £49.50 on the 07.32 to Paddington, valid for return on any train.
Fuel to get to Warwick and back home plus parking will account for £20 before you even set foot on a train (never mind 80 miles' depreciation on the car) - and Chiltern don't offer their £6 singles until the end of the morning. Nor are they available on a return journey just after 17.00.