One of the stipulations of being a shareholder is that you may be forced to sell your shares under certain circumstances. Nothing illegal about it, even if you don’t like it.
Takeovers of UK-listed companies are subject to the Takeover Code [0] which is administered by an independent body called the Takeover Panel. The Takeover Code is actually a surprisingly readable document which sets out all the rules that the bidder, the target and the shareholders must follow. A "mandatory" takeover in the UK is triggered when a shareholder goes over a 30% shareholding - they are then obliged to make an offer for the stock that they do not own at the highest price they have paid in the previous 12 months.
Shareholders cannot be forced to sell until the bidder has received acceptances of more than 90% of the shares to which the offer relates. The board of the target will offer shareholders an opinion on the takeover price - they can either recommend or reject the offer. Typically, when boards recommend an offer then shareholders will accept but there is certainly no obligation to.
Interestingly, the Takeover Panel used to have no legal enforcement powers (I'm not sure exactly what their status is these days). To ensure compliance with the rules there was a punishment called 'cold-shouldering' - basically if you breached the Takeover Code in an egregious way, the Takeover Panel could instruct market participants to stop dealing with the guilty party. This has only been used in very rare circumstances [1].
[0] https://www.thetakeoverpanel.org.uk/wp-content/uploads/2022/...
[1] https://www.thetakeoverpanel.org.uk/the-code/compliance/cold...