BMW or self service depends on the age of the car and value. Forgetting the convenience for a moment the extra costs of BMW service simply does not work on older cars. If the car were a newish example then perhaps, but do the maths on an older one over many years and it would never realise a price high enough to cover the incremental costs. Even BMW appreciate this by dropping costs based on age.
With Zeds in the £6k region and up to 10 years old all those services at £00's won't reflect a jot in the value, perhaps in some cases just easier to sell. I'd buy an older example on condition, receipts and my examination of it, not a series of stamps in a book. If it were am ///M or 1-3 year old car,then for sure I'd look for history - dealer or Indy.
Edit - I should add I opt for a good local Indy at excellent rates for I and II and do everything else myself inc in between oil and any repairs, replacements, etc.