At what point on the journey from Billericay to Liverpool Street is the ticket likely to be frowned on. Would it fail to operate the gate at Billericay or at Liverpool Street?
It's impossible to say, really; the automated analysis of paper tickets at gatelines isn't terribly clever and generally wouldn't be able to reliably accept or reject tickets for a complex, timed journey of that nature; if the barriers don't open, you'd be relying on the goodwill of any gateline or onboard staff who saw that the ticket was an Advance one, and decided to ask for the reservation coupon. They're more likely to be filtering by locations, ticket type (Advance/varieties of walk-up) and discounts (child/Railcard) - and of course the date of travel. Therefore the ticket could still be rejected even if it was valid for the time of day, if it was discounted or of a certain type. If the correct time is stuck to, and it's then passed by a manual inspection, that's fine. However, the same ticket could equally be rejected by the barriers at the wrong time of day as well, and then an inspection could result in travel being denied, or worse.
I suggest that a manual check of the reservation coupon is more likely to occur when entering a mainline station, or onboard mainline services, but you can never be sure.
Pausing your journey within the station itself for half an hour to an hour would be perfectly reasonable - for example, if you've just travelled in good time for your booked leg of the journey, and want to get some food on the way - but it may be a problem when it extends much further than this, or if the passenger leaves the rail network, in this case including a route across London.
Often, cross-London journeys can be difficult to track for revenue staff, as there are often several different, valid routes across London for each ticket, or they may take significantly different amounts of time, or they may involve crossing roads or busy concourses between Tube and mainline stations. For example, some journeys may be much longer than others if you're limited to the sub-surface lines (eg. Circle/District) because you've got a non-folding bike. However, this is no excuse for breaking your journey for several hours halfway across town, and questions might still be asked.
In other words: break of journey (leaving the network/stations en-route) is not permitted, and whilst you may get away with it around London or on the Tube part, railway staff often have their ways of spotting things which are wrong, and it could quickly get you into trouble.
(...by "you" I mean a passenger...)
