The UK is still supposed to be a free country, where people are allowed to hold and express their opinions. With a very few exceptions, there's no legal right to not be offended.
The content on that billboard is a statement of truth, displayed with a political intent, which is entirely legal and being able to display it is something everyone who believes in a free society should support. You may not agree with the message, but if you don't support the legal right of someone to display a legal political message, or speak a personal opinion, then don't complain when your own message, and your civil rights, are suppressed.
Remember Martin Niemöller's sermon; it's just as relevant today. Also, according to the OED:
Bigot
noun
A person who is intolerant towards those holding different opinions.
The intention of the billionaire investors behind this sudden anti-women campaign is to exploit the medical needs of trans people for profit. Trans people with genuine needs for help and support, are being exploited for profit. See Jennifer Pritzker and others like him. Women and our rights are being suppressed because we get in the way of greed. Without words to describe correctly what we are, we disappear. You can't speak about something you don't have words for.
Research the 'educational foundations' these big investors support, teaching in a way that swings the debate towards the needs of trans people while eliminating all differing opinions. Research how much big pharma in the USA spends buying legislators' votes. You'll find this as difficult as I did, because when limits on private donations to political campaigns were removed in 2010 (the Citizens United case), it was legal for donors to hide all their campaign spending. Jane Mayer's Dark Money is very detailed on this and much more that's relevant, it's worth taking the time to read if you can. And want to, of course.
Earlier this week I posted in a national newspaper a comment critical of Adrian Harrop; it received a fair number of recommendations. The automated email I had from the newspaper told me it had also received a number of comments which had all been deleted because they breached the newspaper's terms of publication. Presumably they didn't agree with me. The number was in double figures, I can't remember exactly how many because I've deleted that email, it's not the kind of thing I need to keep. I received this email stating how many comments on mine had been deleted, within 10 minutes of my posting in the newspaper. There's a whole army, presumably paid by vested interests, or an automated system, probably both, ready to try to squash any opinions contrary to their interests.