Before starting to trade you should consider arranging a public liability insurance policy for engineer to protect yourself should you be held legally liable for things such as injury or property damage during the course of your work. You do not legally need to have public liability insurance but it is recommended since one claim could easily put you out of business or worse leave you paying a very large bill for a long time.
If you have any employees then it is a legal requirement to have employers liability insurance for engineer. You can usually obtain a quotation for a combined public and employers liability policy. Failure to not have an employers liability policy in place could not only result in a fine or custodial sentence but also huge legal costs should your employee hold you liable for an otherwise insured peril.