May I also throw into the discussion the use of trainsplit.com. If you've not used it before, it does what it says on the tin really; it splits your journey up into multiple tickets, and saves you money. I've just looked, and there's a journey opportunity commencing at 12:02 from Bicester North, via Marylebone, Kings X, Edinburgh and Stirling, currently costing £84.62 (a saving of £109.98 over the Anytime Single).
As this is for work, I'm assuming you'll need to make the journey a few times so may I also suggest you look at the range of RailCards on offer. If you're eligible for one, you'll pretty much get back the initial upfront cost of it from this journey alone. You'll also need a reservation to travel on LNER up to Edinburgh, which is fine if you book in advance as it should give you one automatically, assuming there's room on that particular train.
I would also carry your lanyard and badge just incase you get stopped at any station - whilst this has died down recently, police still actively patrol major stations stopping any unnecessary travel.