You are asking a stranger to let you into their house, often at night, often when they are already stressed. Trust is not decoration here, it is the product.
Be careful and be accurate about certification, because getting it wrong is worse than saying nothing. Gas Safe Register is the official registration body for gas work in Great Britain, the Isle of Man and Guernsey. Registration is a legal safety requirement for anyone carrying out gas work, not a quality mark or an award, and it does not cover general plumbing, bathrooms or drainage. So if you are registered, publish your registration number and let people check it.
Do not describe it as an accreditation you earned, and do not imply it covers the bathroom you fitted last week. Customers who know the difference notice, and the ones who do not are being misled.
The same discipline applies everywhere else. Public liability insurance is worth stating with the cover level. Employers' liability is a legal requirement if you employ anyone. If you belong to a plumbing or heating trade body, name it exactly and link to your listing so it can be verified. If you hold water regulations or unvented hot water qualifications, say which. What you must not do is scatter unexplained logos across the footer, which is the single most common way a trade site quietly loses credibility with the people who actually read.
Then there is the ordinary human proof, which matters at least as much: your face and your name, a real address or at minimum a named town, reviews shown with the reviewer's area rather than as anonymous five stars, and photographs of your own vans and your own work rather than stock images of a smiling man in a clean shirt.