Thanks for the advice all - I think I understand the concept and how it works now!
Obviously the context of this is that I'm looking to minimise the cost, to me, of my commute by taking 'advantage' of the fact my employer covers the cost of a z1-6 travelcard as I move around sites on the days when I'm in london.
I take the point about it being a taxable benefit in kind ....
I agree I wouldn't worry about this. The employer hasn't bought that travelcard to subsidise your commute, based on what you've told us, they've bought it because they need you to travel around between central London offices as part of your work.
If you bought your ticket from Cambridge to London Underground zones 1-2, or if you bought a ticket to London Terminals and then paid your own tube fare to get to the office and to get back to King's Cross after work, and only used your travelcard for its intended purpose of going between offices during the day, that wouldn't change anything regarding your tax affairs. Train tickets are nil-rated for VAT, so HMRC wouldn't even gain any revenue from the extra tickets you'd bought.
They won't be concerned if you've benefited from that ticket for a small part of your commute, as long as the benefit is incidental.
It's also worth asking yourself if they'd mind if you bought a ticket to London Terminals but then used the travelcard to take the tube to your office? If they wouldn't object to that (which they wouldn't) then I don't see how they can object to you also using it for that part of your train journey that is covered by the travelcard.