I had a look further into this, because I wanted to better understand what factors might cause an act to be considered indecent exposure (or outraging public decency). This led me to some guidance on naturism and other non-sexual nudity, from the crown prosecution service.^1 It appears that having an "intention to cause alarm or distress" may be relevant for protests like this — arguably the entire point of the protest is to use the shock value of the nudity as a protest.

That being said, I think it's a bold move and possibly an effective protest. Even if public indecency laws are gender neutral, it would still be a strong message if any of these women got arrested for this — the reason why these women are capable of causing alarm or distress by going topless is because these are "female presenting nipples" (to use a heavily-memed phrase from the Tumblr porn ban era)