I am surprised you could be tricked into admitting you wouldn't have bought a ticket if nobody had been there, as the OP presumably HAD to buy that ticket and would have bought that ticket at the ticket office or a TVM (making the last bit of the previous journey paid for).
If I was asked, clearly I wouldn't have said 'if you weren't here, I'd have got away with it' as that wouldn't be true! And if I've asked to buy a ticket at the gate, where's the intent? You've asked just as you have before.. how do you know/care that these people have a different uniform and name badge on?
Now, if I was going to Newcastle and then using the Metro, could I buy a ticket from Hatfield to cover me on this, or would I go and buy a ticket when I arrived at Newcastle? I'd almost certainly seek to buy the Metro ticket when I arrived and not necessarily think you could buy it from any station. Brighton is quite far away from London, even if it happens to be covered by two suburban operators - each with their own competing tickets.