Glad it helped- I found the need for Employer's Liability Insurance to be the biggest surprise, so wanted to flag that up to people.
For example, my contracting company took out a policy for the Professional Indemnity cover, but it also comes with some Public Liability and sufficient Employer's Liability cover to meet statutory obligations if we take on employees.
I don't know whether this works out cheaper than finding separate policies for exactly what you need, so obviously please do your homework if you're in a position of needing this type of cover.
EL is the only compulsory insurance which applies to all private sector employers, although in certain industries you will have additional requirements (e.g. conveyancers have to purchase professional indemnity insurance).