During my research for the "field uniforms" of the British Army 1815 I came across a note the acknowledged researcher Rev. Sumner left in his notes / sketches, now in possession of the ASKB.
He wrote in the section covering the Foot Guards: "Many men of the Foot Guards at Waterloo fought in their Surrey Militia jackets (Cotton, "Voice from Waterloo", page 10).
The mentioned text of Edward Cotton in his memoirs is written on page 7 of 1854 edition (can be downloaded via google) - he states that "a large proportion of the British troops was composed of weak second and third battalions, made up of militia and recruits, who had never been under fire*..." - the footnote says "* The 3d guards and 42nd Highlanders had near eight hundred militia-men in their ranks. The guards actually fought in their Surrey militia jackets."
Of course I checked several other sources - e.g. the three-part study of Bryan Fosten published in the series "Soldiers of the Napoleonic Wars" of Almark in the 70/80s.
Bob Burnham and Ron McGuigan give in their exceptional study about the British Foot Guards at Waterloo some strength numbers for the four battalions present in the Netherlands - with the addition of reinforcements; for the period of March to June 1815, the following numbers can be derived as "new" soldiers to the guards battalions:
2nd bat. / 1st Guards: 79 men
3rd. bat. / 1st Guards: 35 men
2nd bat. / Coldstream Guards: 264 men (!)
2nd bat. / 3rd Guards: 274 men (!)
So particularly the Coldstream and the 3rd guards got lots of reinforcements, most of them in April/May 1815.
Now the very interesting question: do you have any more hints except the quote from Edward Cotton that the foot guards (particularly 2nd and 3rd Guards) had a significant number of soldiers fighting at Waterloo in their (Surrey?) militia jackets?
This would give figure dioramas surely a nice additional note 😀
Greetings from Berlin
Thank you all for your comments - indeed the coats of the Surrey Militia is quite similar to the Guards uniform due to the royal blue facings - and the prescribed pairs of buttons/laces would make it difficult for witnesses to differ them from at least the Coldstream Guards - but for the 3rd (Scots) Guards it seems to be more difficult.
I appreciate David's opinion about the regiment's tailors to change the laces and buttons - but did they have the time AND the material to do this in time for the reinforcements reaching the Netherlands especially in May 1815. I also think that only one source is quite "dangerous" to take it as proven, but perhaps Cotton got in contact to soldiers of the Guards and they told him about this feature - at least we don't know and I fear the only answer may be by consulting the archives of the Guard's regiments.
@Andrew: good point and yes, if they came from their barracks onto the Netherlands they would have had their guard's coats - unfortunately the numbers given by Bob and Ron in their book don't differentiate between the provenance of the troops.
Greetings from Berlin
Worth noting with respect to the numbers that the 2nd battalions of the three Guards regiments had initially deployed to the continent understrength in December 1813 (2/Coldstream and 2/3FG especially so), leaving some companies of each battalion behind in London, and were slowly brought up to strength thereafter. It is therefore not a given that men joining the battalions on active service were all new to the regiment.
In case there exist comments that part of the Guards did wear militia jackets - seemingly the observer must have recognized them as such and not as the usual Guards jacket - so - in my view they did wear such and there weren't transformed into Guards jackets by new lace and buttons.
The Royal Surrey Militia are show. With blue facings, so just button and lace would need changing
An interesting thought, but the designs were broadly similar, with the exception usually of facings, lace and buttons. Being household troops, Guards had dark blue (more like black in reality) facings, as would any ‘Royal Militia’. Changing buttons or lace is an easy proposition. Regimental tailors would be able to do that swiftly and easily, and as the Army had some time in Belgium it is inconceivable to me that units as punctilious about dress as the Guards would not have done so. Despite what some novels and TV shows have allowed us to believe, swanning around in the ‘wrong’ uniform was not as easy as that. So they could well have fought in their militia jackets as Cotton said, but with appointments to make them look functionally identical. I still wouldn’t rule out Waterloo Uncovered digging up the odd Militia button though. After all, in the British Army uniform is a noun, not necessarily a adjective.