I make this journey a fair bit, and have to say that all the options are pretty much as inconvenient as each other.

Note that between 23/12 - 01/01 the Stansted Express will operate to and from Tottenham Hale, not Liverpool St. This makes it a good idea to avoid London if you're travelling during that period.

http://www.stanstedexpress.com

The East Coast Mainline itself has engineering works over the holidays too, so it's worth checking before you commit yourself.

I usually use easyJet, but you say that's not an option.

Have you thought about a one-way car rental? Europcar and Avis have some keen one-way rates.