The "British Isles" community on the postcrossing forum is great, I've met so many lovely people there! I occasionally participate in RRs there as well (with the added bonus of UK postage being cheaper than sending cards abroad :p). This very nice postcard arrived yesterday, it's from an RR and from Serena (again! :)).
I'm quite embarrassed to say that it took me this long to find out more about this picture. For some reason I always thought it might have something to do with the Carry On film series... *hides*
Anyway, apparently, 'Keep Calm and Carry On' was a propaganda poster produced by the British government in 1939 during the beginning of World War II, but never used. It was rediscovered in 2000 and has been re-issued by a number of private sector companies, and used as the decorative theme for a range of other products (like postcards here :D).
Serena always uses very nice stamps on her postcards, and this was no exception. This one is from a set of 10 stamps issued in 2009, depicting British design. This stamp shows the Mini Skirt, designed by Mary Quant.