There are two ways to think about rights: there are legal rights and then there are human rights. Legal rights are conferred by some piece of legal document (legislation, constitution or common law) that a person is able to seek legal redress if their right has been revoked or diminished. Then there are human rights - what we as individual humans believe that each humans should expect as a basic right. The two are not always aligned, predominately because human rights vary greatly from one person’s interpretation to the next.
I think what the company is probably (accurately) arguing is that there is no legal right to swim in the UK, as no specific document states this with any specificity, so the complainant isn’t due compensation or redress of behaviour under the law. This is what the courts will examine as they are the interpreters of law but not the creators of law.
Now, does she have a human right to swim there free of sewage? I damn well think so, and I don’t think that would be a controversial opinion either. The problem is that what we think the law should be and what it is are often different, because legislation can’t represent every view simultaneously. There’s no law that could be drafted that makes forced birthers and pro choice people agree - someone will always lose out.
All of this is to say that while fighting this in court is a shitty thing to do (pun very much intended), it makes sense based upon the way our legal system is set up. There is no incentive for private business to respect rights that are not legally conferred, but there is a financial incentive to do the ‘cheaper and technically legal’ thing. Until we overhaul our legal systems to be inherently protective rather than inherently exploitative, this behaviour will continue.