There may be a flaw in your scheme.
Over the last couple of years, Network Rail has shut half the lines in the UK completely over the Xmas period, for engineering works.
You may find that the services you need, will not exist during that week. Check before you buy your pass.
EDIT: a search on the Networkrail.co.uk site for Christmas brought up this and more:
The West Coast main line will be closed between London Euston and Hemel Hempstead after the last trains on Christmas Eve until Monday 29 December 2014. It will also be closed between Stafford and Crewe after the last trains on Christmas Eve until Sunday 28 December 2014.
Between Christmas Day and 2 January 2014, three upgrades at London Victoria, Gatwick Airport and a major junction between Redhill and Purley will take place which will impact Southern, First Capital Connect and First Great Western services in the area.