The thing is the Royal Mail's guarantee is worth sod-all. Anyway, last mailing dates are as follows. You might get stuff before Christmas that is sent later than these dates, but it's less likely.
Monday December 6: North America, Australia, Japan, NZ, South America
Friday December 10: Eastern Europe
Monday December 13: Western Europe
Tuesday December 21: UK
Remember you've got to leave time for us to process the order too. Thanks