Look there is NOT legislation, law stopping you from setting up your tripod, the more you doubt yourself and others the more you play in to there hands, common sense is what you should apply, if you start holding up masses of traffic without allowing them to pass that is not fair, but if your tripod is a normal one that can be moved and is not some permanent structure then you are in the clear, and should pay no heed to them, you should ask them to prove it first, and as they cannot then click away, I feel you always should remain polite but firm, and state clearly they are infringing your rights, by there behaviour, and that they are acting outside the law.
If you state there behavour to you is offensive and not law abiding you have basically turned it on its head.
Steve