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 😀