Search the public Companies House register at find-and-update.company-information.service.gov.uk by company name or company registration number (CRN). The registered office shown there is the address a document can validly be served on the company at, under section 1139(1) of the Companies Act 2006.
The registered office is the official statutory address — not necessarily where the company trades. Once you have confirmed it against the live register, instruct a process server to attend and serve. Tracenet verifies the address as part of every corporate instruction, so the attendance is not wasted on an out-of-date address.