I was visiting Edinburgh only last weekend, and chanced to make a few purchases at a local Co-Op store. Obviously spotted the wording on the carrier bag "at the heart of Scottish communities since 1859". Scotmid didn't operate until 1981, and the 1859 is the year of formation of St.Cuthbert's Co-operative Association. The rule is you always pick the date of the oldest Society in any amalgamation. 1859 is the same year as the formation of the Manchester & Salford Equitable Co-op.
The M&SE started when they had 100 members and over £100 in capital, St.Cuthbert's opened with only 63 members and only £30 capital. It took many years for it to grow into a thriving Society.
One interesting fact about St.Cuthbert's is "the directors were, for twenty-two years, not in any way selected, but were simply taken from the roll of members. The names were called out at a quarterly meeting, and the person called upon could either accept or decline office as he thought proper....not until 1881 when capital was nearly £10,000 and the annual sales reached £30,000 did the members choose whom they thought best qualified for the office."
St.Cuthbert's made the last horse drawn deliveries of milk in 1985. This and some other history is in a timeline is at the Scotmid website.
Scotmid 150 years
The quote above is from a footnote in The Consumers' Co-operative Movement by Sydney and Beatrice Webb, 1921, who reference First Fifty Years of St.Cuthbert's Co-operative Association edited by William Maxwell, 1909. The Webb's book is out of print and I use a battered 1930 edition but it is available online at Archive.Org