Thanks Croppie, for pointing out what should be the obvious. Back to Topic, my interpretation is, if it's a HE Boiler LLD or Balanced flue then OK as long as installation complies with BS5410, Part J and of course Part L1b. If its SE or open flue, far too much trouble.
Just because someone else fits them, doesn't make it right.
Any S/H boiler decommissioned and standing empty is corroding internally.... so my advice is leave it alone and protect your reputation as a honest professional tradesperson (PC)