Steam GWR Museum with a special additional inscription. According to the Royal Mail press release:
Post and Go machine A010 at STEAM GWR will carry the locomotive design from the RM Heritage set from 25 February 2016 to celebrate the 175th Anniversary of Swindon Works. The stamp will carry the underprint ‘Swindon 175’ together with the standard ‘STEAM GWR’ identifier. The underprint will run until the end of the year.
UPDATE 15 February 2016
A special postmark (reference 13748) has been announced for the first day of this inscription, and can be obtained from the London Special Handstamp Centre on any stamps/covers (see Postmark Bulletin for instructions)