The first slogan marks the Centenary of World War 1, also known as The Great War (as in the stamp issue) or the First World War (as shown on these postmarks). First, the image as shown in Royal Mail's library:
Now some examples, from Edinburgh Mail Centre 4 August 2014 (thanks Adrian),
Glasgow 6 August and Norwich 7 August
Today's post also had one of the above from Royal Mail Tallents House, with the Joe Mercer stamp.
Unfortunate, really, as the other new postmark marks the centenary of the birth (9 August) of Joe Mercer who featured on the Remarkable Lives stamp earlier this year. Again, Royal Mail's publicity picture first:
Peterborough and possibly Plymouth/Cornwall MCs, showing the two different styles, used on 7 and 8 August 2014 respectively.