Just in case this is of any use to anyone here
I work for the NHS and emailed Costco to ask about membership after reading threads mentioning cheap bikes for sale. They said if I take my NHS ID badge and utility bill, I can join.
Membership is currently £25 +VAT per annum and that gets you and your spouse/partner a card each.
Their application form lists the following criteria for membership...
Current or Retired Employee of:
Banking/Finance
Civil Servant
Education
Fire/Rescue
In-Sewer-Ants

Local Government
Medical/Health Service
Police Force
Post Office
Airline
Qualified As:
Chartered Architect
Optician
Chartered/Civil Engineer
Pharmacist
Chartered Surveyor
Qualified Accountant
Dentist
Solicitor/Barrister/Magistrate/Advocate
Hope that helps.