Contact the HR/Payroll department of the company, they should be able to provide that reference number. It's unique to the company. Alternatively you could state "not known". It's not something the will impact negatively, bearing in mind you provide your NI number as well which is the key to all yo...

Hello all, I am applying for British citizenship (naturalisation) after holding ILR for more than a year. I changed my employer in June 2020. Therefore, I don't have a P60 form with my PAYE/Tax office reference number. Before June 2020, I had worked for a company for 6+ years and I have all the pays...