Might be covered by the Highway Code? 

https://www.gov.uk/guidance/the-highway-code/waiting-and-parking-238-to-252

Maybe refer to Rule 242 and/or 246 but I guess it will depend on how/where they are parked?

There is also some usefulr refernce information at https://app.croneri.co.uk/topics/parking-loading-and-unloading/indepth?product=143 for you to consider

Hi Gill

You won't get any joy from looking for guidance from HSE on this one as HSE does its utmost to steer clear of anytbing to do with OFF site traffic except when it means a clear and obvious interface with live work sites, e.g. highway maintenance. Arguably one of the reasons why the scope of RIDDOR has never been extended to take account of work-related transport accidents most of which are excluded from scope.

Good advice from Acorns.

The other thing that you might wish to consider is which of the roads in your scenarion are "public" and which are "private", not least as it is an offence to obstruct the public highway.

 The difference might seem obvious but not in terms of interpreting road traffic legislation. So, as example your favourite supermarket might own its car park, but the roads to and in the car park are likely to be deemed "public". 

Not my field of expertise, but I think that whether access to your client's premises is "public" or "private" would be determined by the nature of who is allowed relatively free access to any privately owned road infrastructure. Hence, if your client operates say a hospital complex almost all its roads are likely to be "public" whereas if you get stopped at a gate by security before being let in then may be that is NOT a "public" highway.

Obstructing a PRIVATE highway is perhaps largely a matter for the civil Courts.

That said if there is double yellow line across the entrance, then parking alongside that would be likely an issue where road traffic legislation would kick in.