Remember the 4t Edition Code of Practice is a 'GUIDANCE' document and not a legal one - the retest period must be down to the Duty Holder to decide.
As a PAT tester, you have a responsibility to advise on frequency of testing based upon either historic evidence from previous test results that would suggest either increasing, decreasing or leaving the re-test period for an appliance, or from the Table in section 7, which gives an INITIAL retest for the type of appliance.
After performing your risk assessments or PAT testing, you present your evidence to the Duty Holder for them to make a decision on the re-test period, so that THEY have the final decision on the retest not you.
This is why the code states NOT to put in the retest date on the PAT label - the records and appliance listing held by the Duty Holder will dictate this.
However, there will be customers who will want their PAT testing done every 12-months or annually. That is their decision and if that's what they want to do, then go with it. Just get then to sign an agreement to do this or ask them to put it in writing, then you are covered.
Same as if they still want the retest or next test date put on the label - its the Duty Holders decision and get them to agree, in writing.