Not seen this mentioned on the forum yet, but RM are merging their 25x35x16cm and 35x45x8cm small parcel sizes, into one 35x45x16cm box for upto 2kg. Details say this is until 18/01/2015, so obviously to accommodate the Christmas presents demand.
Full details here:
http://www.royalmail.com/new-parcel-size
(Shame we recently bought 100 boxes for the 25x35x16cm size!)