Hi!
If you can, do get the annual. It's 40 weeks of travel, the last 12 are 'free'. Otherwise do the month +1/2/3 days thing. Overall it works out cheaper. I spent an evening working it all out for the rest of my year, taking leave into account, and figured out a set monthly cost out of my salary - the 'true' cost other than what I was paying every ~5ish weeks which can feel like a lot at one time!
I don't pay for the tube, I walk to Moorgate from farringdon but occasionally 'treat' myself from St Pancras if I'm tired or late. an extra 80 odd quid or whatever per month on the season for packed tubes is just not worth it! Have you considered an annual boris bike thing or even taking your own bike on the East Midlands train to St Pancras? My boyfriend does that every time he stays. Depends if you mind the rain - I do it vary rarely. It is quite convenient though, there is always room on the 7.55 and 8.30.
Good luck with the move, it's a nice town with some nice restaurants etc.