Assuming you're talking about heating boilers and not bolshy old women, it would be a Corgi registered boiler inspector, the bigger named firms seem to offer more 'comeback'. Ofgem is the regulator for both gas and electricity.
In hired premises it is the owners responsibility to get this done annually and they receive a certificate to display saying it has been done , had to have ours done and sent bill to hall owners, surprisingly they do it annually now!!
Hi my hubby is a gas engineer and he says any corgi qualified engineer qualified to do commercial installations can do gas boilers, any OFTEC engineer can do oil boilers and as its commercial there must be an annual inspection and it will probably be a condition of the buildings insurance policy{this is for a preschool for example- in a private home its still a very good idea to have your gas boiler/oil boiler inspected and serviced anually] hope this helps!