To choose the best loft insulation installer, you need to consider a range of factors. The best place to start is by checking the certifications and accreditations of your chosen installer. Professional companies will be recognised as such through quality approvals from organisations such as TrustMark, or the Common Assessment Standard (CAS).
You can also look at what national trade associations the installers are a member of, their reputation with previous customers, as well as existing warranties and insurances they provide.