In the UK there are no legally-binding regulations for service agents to follow by law. However, OFTEC (Oil Fired Technical Association) does provide peace of mind for consumers. They provide a directory of members who are approved to install and service oil equipment, and non-compliant work will be investigated.
It is also important to ensure that an oil boiler installation must comply with Local Authority building regulations, and an OFTEC registered technician is allowed to self-certify their work - so you have no paperwork to deal with. You are given a certificate confirming that the work done meets the relevant Building Regulations, and you need to hold onto this in case you need to sell your property.