If someone in UK keeps getting adverts for products like mobility scooters through the post, where would they be getting their name/address?

Answer 1

Almost anywhere.  Many places sell and resell lists of names. 

Answer 2

Register for the mail preference scheme to stop junk mail. They get names from other retailers and online companies that record what you buy, the electoral roll, life insurance companies etc basically anyone you give your data to, and then they profile people often by age and location.