Problem  is that different people mean different things when they say self employed. It is quite possible for someone to be regarded as self-employed for tax  purposes but to be treated as an employee for H&S purposes. It really depends on the level of control that the employer has over the employee. If the employer gives detailed instructions as to what to do,  how to do it and when to do it, then they a re probably employees rather than self employer.

As their "employment" is in pursuit of your companies business activities they should be following your procedures.

Pretty sure you would not be permitting free reign in their behaviours to others when representing your company

Are the self-employed officers going to be working in parallel to your employed officers doing the same types of things in the same ways?  Is it just the contract that they are working under that is different, while the work is the same?

If so then as far as H&S (not employment rights and tax liabilities) is concerned then they are probably in effect employees and should be treated the same.

Here are some more ways of looking at it.

You suggest that what you want is for the risk assessments and procedures for the self-employed officers to mirror the risk assessments and procedures for the employed ones.

If you asked them each to create their own risk assessments and procedures in the hope that they would match yours, you could be disappointed.  Either they would be guessing what you want the risk assessments and procedures to say (and are unlikely to guess right), or they would be copying out the ones that you showed them (which would be a colossal waste of time).  

Apart from anything else, it is just far easier to tell them "these are our risk assessments and procedures and we expect you to work to them."

Your risk assessments probably contain control measures that you as an employer have power over but which individuals don't.  For example (hypothetically), there could be a control measure for the risk of violence involving providing information that certain jobs are red flag jobs where certain extra precautions are needed, based on history of related jobs.  You would know which are the red flag jobs but they have no way of knowing. So it is a control measure that you can implement but they can't.  In that case it makes far more sense for you to take responsibility for the risk assessment.

Presumably there is some form of code of conduct about things like dress, politeness towards the public, not bringing the company into disrepute,  etc. which applies to your employees and which you equally expect these other representatives of your company to sign up to.  What applies to a code of conduct, applies to safety standards too.

A construction company I worked for used self employed trades people both on the tools and in a management capacity.  They followed company policy and procedures and were not issued PPE.

There came a time when the HMRC contacted the company about the same self employed being 'employed' for many years and they suggested they should be PAYE.  The company won the argument with HMRC and kept them self employed.

I suggest you treat them the same as your PAYE employees as far as anything to do with work is concerned.