Wouldn't intending to break the conditions of the ticket to make a financial gain be fare evasion or even fraud? OK if he's intending to ask regardless - but what if the gates are open and he walks through without asking? The moment he steps outside the station without permission wouldn't that be breaking the journey and thus against the ticket conditions, at the very least a bylaw?
What an accusatory and very unhelpful response. Hopefully, they won't encounter anybody like you, whatever their course of action. It is this sort of post that discourages me and others from even reading this section.
They bought the ticket in good faith, but subsequently discovered there were some additional restrictions on top of what they expected. Arguably, this is their own fault, as the terms are available to see, but given the tickets are being marketed as off peak day returns, additional restrictions would not necessarily be expected.
The "no break of journey" restriction is well hidden, as it is contained at the end of the section "Are any journeys excluded from this offer?", which is primarily about blocking use solely within London and to/from airports. I can't find any reference to not being able to terminate a journey short. Additionally, in the section "Are there any other restrictions?", it states "Normal off-peak return restrictions apply" and "You dont have to keep to booked trains, but you do need to remain within the time restrictions."
I would also suggest that the poster was intending to make a journey to "London", rather than Liverpool Street station in London. In the case of a big city like London, I would expect it is quite common for people to use different stations in each direction.
The reason given for wanting to terminate short was a slightly easier onward Underground journey. The additional benefit of a cost saving was with regard to that Underground journey - nothing to do with the journey to London with Greater Anglia, for which the fare is the same to either station anyway.
As it happens, the cost saving on the Underground doesn't exist anyway! From Liverpool Street, the cost is £2.40 Oyster/Contactless and £4.90 cash at any time, whilst from Tottenham Hale, it is £2.80 off peak or £3.30 in the morning peak for Oyster/Contactless and £4.90 cash at any time.
I will add that these specific promotion tickets are very confusing. Despite being clearly marketed as flexible off peak day returns, on Monday, when I bought one from London to Harwich Town, the ticket printed as "Advance Single" with "Outward Coupon 1 of 2" and "Return Coupon 2 of 2" and no reservations, just the phrase "Valid on Greater Anglia services only".
As I was using reservable Norwich trains to/from Manningtree, I was fully expecting to be asked for my (non-existent) reservations. As I had plenty of time to spare, I went to a ticket office counter at Liverpool Street station and asked them. The first person I spoke to didn't know anything about the tickets, so called across a colleague, who told me not to worry and that it would be fine as "these tickets are off peak returns" (despite no reference to that on the ticket) and (incorrectly) "you can't reserve trains on that route, so there's no issue". In the event, on my outward train, the guard barely even looked at my ticket, whilst on the return, there was no ticket check at all, so it was all moot!