There are problems of working out when you might find a slogan in your mail. Royal Mail press releases often suggest that a slogan is being 'used' on the dates that they are actually intending that it is delivered. Logically for first class mail a slogan which they suggest will appear on, say Wednesday 10th will have been used on Tuesday 9th - or maybe on Monday 8th on 2nd class mail.
If you want just a single example and a slogan is in use for a week, this is not the problem that it is if the slogan is said to be only 'in use' on one day. If the slogan ties in with a new stamp issue and you want a first day cover with the slogan, then the date is crucial! (And there are other difficulties.)
Royal Mail's press release for the Magna Carta stamps read: "The Special Stamps postmark will run from Tuesday 2 June to Friday 6 June and will say ‘Magna Carta stamps - Commemorating the foundation of liberty’. While the second will appear on Monday 15 June and will say; ‘Magna Carta 800th anniversary’.
For the Battle of Waterloo stamps, they wrote: "The anniversary will also be marked with a special postmark on all stamped UK mail on 18 and 19 June. This will read ‘Battle of Waterloo 1815-2015’."
For Armed Forces Day on Saturday 27th June "a postmark will appear on stamped mail within the UK on Saturday 27th June. It will say ‘Armed Forces Day, 27 June'."
The Magna Carta stamps were issued on 2 June, and the 'stamps' postmark duly appeared at the end of May!
Gatwick IMP (Integrated Mail Processor) - may be in reverse order on square envelopes.
Norwich iLSM (Intelligent Letter Sorting Machine) - does not show Royal Mail cruciform.
This was recorded used at Gatwick on 2nd class mail on 30 May and 1st class mail on 2nd June.
This list (obtained from another Royal Mail source) was provided to me by a reader on 11 June:
17 June only: Battle of Waterloo 200th anniversary
18 June only: Magna Carta 800th anniversary
25-26 June: Armed Forces Day
27 June to 3 July: Dog Awareness Week
So what in fact do we have? No evidence yet of when the Waterloo postmark was used. The Magna Carta 800 postmark used at
Gatwick on 13 June on 1st class mail,
Southampton, Birmingham and Gatwick on 15 June on 2nd class mail,
North and West Yorkshire on 1st class, and Gatwick on 2nd class both on 16 June.
UPDATE 22 June 2016:
Waterloo slogan now received from Edinburgh on 18-06-2015 (thanks Robert!).
Chester was (still or again?) using the Royal Baby slogan on 16 June.
And there was a surprise from Edinburgh, a Father's Day 21 June reminder used on 18 June! (Two reports of that, both about Edinburgh MC - thank you!) Which means that Edinburgh used two different slogans on the same day!
Belatedly we have examples of the Father's Day postmark from Norwich, Peterborough and South Midlands.
The first Armed Forces Day postmark has arrived, from Sheffield Mail Centre 25.06.15:
And from Mount Pleasant in the other format:
First reports of the Dog Awareness Week slogan, on 27 June from South East Anglia (2nd class) and Jubilee Mail Centre (1st class). (Thanks to AB and EB).
Further reports of any of these - announced or unannounced - will be welcome!