That is a rather open question and it's not really the place to go recommending companies, but to get you started... Assuming your project is in the UK most, but not all, EHV connection projects involve Lloyd's registered / NERS accredited contractors for some of the DNO-side. This provides some level of quality assurance etc since they're licenced to build systems to be adopted by the DNO, and it's therefore not uncommon, but by no means required, to use the same contractor on the customer's HV/EHV network too.
Do note, however, that many competent consultancies are not Lloyd's registered; all it is is a list of firms who have specific quality and safety systems and competencies in house to deliver to DNO specifications. Many will use subcontractors and specialist consultants to actually deliver.
