I understand that the revised Chapter 3 will have much the same recommendations on sign placement and spacing, but that it will be much clearer that these are just guidelines and not absolute maximums. The word “should” is therefore likely to be removed with respect to the “15 m” advice you mention.
In the meantime, all you really need to do is document your reasons for departing from Chapter 3 and have that available to send in your evidence to TPT if necessary. Provided you show you have read Chapter 3, but have departed from it for a good reason (and that the road user is not unduly disadvantaged by your doing so), then you should be OK. Even for ‘statutory guidance’ (which Traffic Signs Manual is not) it is usually sufficient simply to ‘have regard to it’, rather than to follow it precisely.