Ben,
lt might be more helpful if you asked the specific question rather then generalising. As stated above any pressure vessel / system commercially manufactured in the EU or for importation and use within the EU has to comply with the Pressure Equipment Directive and be CE marked. That is the law. CE marking will be overseen by a notarised body such as a classification society (Lloyds, DNV, BV etc) or an insurance company.
The test code for miniature boilers is exactly what it says, a code of practice not legislation. It is principally designed to bring hobby produced boilers into alignment with the PED and provides assurance to the insurer that the boiler has been constructed and tested to the agreed requirements of the code. It can be used to provide ongoing certification for use of a commercial CE approved boiler. It cannot be used to certify a commercially produced boiler for sale or underpin CE marking.
So what would you reasonably expect when buying a commercially produced boiler? Firstly it must be CE marked, secondly it will have an initial shell test 2xwp pressure certificate issued by an authorised body (could in some circumstances be the manufacturer) and thirdly it should come with a document package detailing materials, construction and maximum working pressure. It won't come with a written scheme of examination, a periodic 1.5wp certificate or steam test certificate.
Not sure if that helps, if you are more specific, more help may be forthcoming.
Paul.