Use of public toilets should not be a problem provided that you use them discretely. If you suspect that the toilets are not connected to mains drainage do not use.
In addition, of course, leave no traces. If you currently use 'blue' products in your loo I would recommend you stop. In preference use nothing if you have a SOG system, or bio washing tablets or liquid.
Given the above, and the scarcity in the UK of public authority provided disposal facilities, including these in the POIs seems somewhat pointless ...
Remember - POI additions and updates can be sent by email to POI@wildcamping.co.uk