I've been looking into this for a similar type of business. If I understand it correctly (and welcome any correction by better informed people):
Public Liability Insurance: in the course of giving advice, your pen jumps out of your hand and stabs a passing brain surgeon in the hand. PLI covers you so that you're not left without a penny.
Professional Indemnity Insurance: in the course of giving advice, you make a recommendation to a client that only a complete idiot would make, and it ruins their personal life and their career as a surgeon. PII covers you so that you're not left without a penny.
For a counselling business, I would definitely go for both even if they're pretty expensive (as they were for me) – but I am the worrying type.