Ok, some initial thoughts. I’m not an expert here so this is just my personal opinions.
In the above statement that I’ve highlighted in bold, I think you need to re-word the sentence to ‘We will never share your details with a third party’.
That should resolve the ambiguity in the current wording.
Personally, I don’t think you can ask them to phone you to opt out because if they don’t phone, you are effectively arguing that they have given consent by default. However, it’s new untested legislation so difficult to predict how it will be interpreted.
Regarding staff taking client details with them, it is your legal responsibility to ensure this doesn’t happen. You can’t just shrug and say ‘what can I do?’. You need to make accessing client data part of your staff training and written policies. Be clear that staff copying client details for their own purpose is classed as a criminal offence and also counts as gross misconduct, which allows them to be sacked without following the normal procedures for dismissal. If staff do take client phone numbers with them and a client complains to the ICO (unlikely in most cases but it only takes 1 disgruntled client to make a fuss), the ICO could prosecute your salon for allowing the breach to happen.
This really isn’t anything new and was certainly the case under the existing Data Protection legislation.
Whilst some argue that the new legislation is designed to curb the big multinationals from illegally sharing and exporting personal data, it seems likely to me that small companies could become easy targets for prosecution if they are especially lax with their basic operating procedures. However, I would hope that helpful advice on how to improve their systems would be the starting point of any action.