It’s not the Police job to stifle the legimate rights of an individual or a group to protest.
It is generally accepted that in the course of protest certain low level offences are likely to ocour which in the normal course of events would be enforced but in a protest are allowed. (Section 5 of the Public Orderr Act and Obstructing a Highway being prime examples). The reasoning behind this is that the rights to protest as enshrined in Articles 9,10 & 11 of the Human Rights Act are so important that a certain amount of Leeway has to be allowed. If you can’t tell an MP what you think outside Parliament where can you tell them?
It’s also an interesting point that with a Majority of only 839 votes in an area that voted 55% to leave she really should be looking at her constituents wishes if she wishes to be re-elected.
Edited to add that “insulting” was actually removed from the offence by Theresa May as Home Secretary so it’s only abusive or threatening words which it now applies to.