Leaving employment to go self-employed in same field, can you take your clients?

My husband is employed as a financial advisor (based in England), he has been there for over 4 years but has no contract (due to how poorly managed the company is). He wants to go self employed for varying reasons and he has lots of great clients in which he's confident would want to follow him.

There is no contract clauses which is great, but what are our options in terms of contacting the clients in future? He was planning on doing a 'goodbye' email from his current company email the second he leaves with details of his own business, but of course plenty of these might not be read.

Can we include something along the lines of, if you DON'T wish for us to contact you in the future, please respond to (new business email) and you will be removed from all future correspondence? Would that protect us?

His only other thought, is calling the clients just before he leaves and asking for their permission to use their details in future? Obviously risky as the owner could find out, but surely at that point the worst thing they can do is sack him with no notice?

Any ideas/advice would be hugely appreciated.