Did you work for the Company or a franchisee? If company then you need to contact the head office. You may look at your appointment letter and find the HR manager details?
If you have worked for a franchisee then go to the franchisee HR manager (where you get your payment from) and ask him/her to issue you the letter. The letter must be on Mcdonalds official letterhead. Business/Store manager do not have this letterhead so you must get it from HR manager/officer.
Tips: Google "UK Mcdonalds shift manager duties"- you should come up with a flyer which mentions all the duties. Prepare a reference letter by following IRCC guidelines. Then email it to the HR manager. I had very good relationship with my franchisee HR manager as I worked for them about 5 years as a shift manager. She followed my emailed letter. Prepared it on company letterhead, printed and signed it, scanned it and sent back to me. I submitted that along with the payslips, p60 and p45 of all 5 years.
Good Luck!
IRCC guidelines for Job letter-
The following documents are mandatory for each work experience declared:
a reference or experience letter from the employer, which should be an official document printed on company letterhead (must include the applicant’s name, the company’s contact information [address, telephone number and email address], and the name, title and signature of the immediate supervisor or personnel officer at the company),
should indicate all positions held while employed at the company and must include the following details: job title, duties and responsibilities, job status (if current job), dates worked for the company, number of work hours per week and annual salary plus benefits;
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/publications-manuals/operational-bulletins-manuals/permanent-residence/express-entry/applications-received-on-after-january-1-2016-completeness-check.html