You don't expect to get car parts back from garage when you get it repaired so why would you with the boiler?
If you ask for them,you can get them. Some people do because there is a common practice of charging for parts that are not fitted, or replacing parts that are not defective. When you get a tyre changed, you are now charged for the disposal of the old tyre. It is not compulsory, you can keep the tyre, it's yours after all.
It's the same with plumbing. There is widespread distrust due to persistent fraudulent trade practices.
In both cases, the old parts belong to the customer who paid for them. You can put anything you like in the small print, it does not become your property unless you buy it or you are given it. The customer bought the old part and bought the new part; end of. The only reason why an old part would end up in BG packaging is if the technician left the BG packaging.
