By legal, I am not talking about suing them but rather that you might look for Ofcom, which appears it might be your FCC equivalent. I would talk to the Nursery first; since, I doubt they are intentionally doing this as a personal attack on you; instead, someone probably saw a checkbox that said something like, "protect against rogue access points," and it sounded like a good idea.. Perhaps ask to speak with an IT person. If I was their Network Administrator and got a call from a Home Owner who described the situation you do, I would immediately log-in and turn that feature off. Most IT folks are reasonable. Where you run into problems is when you run into that guy who needs approval from six different departments and change management before he will remedy the problem, or that guy who's job is not doing anything
If IT does not help, I would speak to someone at the management or executive level or write them.
Next, would be a complaint to your Government body that regulates the airwaves. In the USA, that body is the FCC. Although they have fined businesses like the Mariott for doing things like this, I am pretty sure it starts with a quick investigation and a sternly written letter. In the UK supposedly Ofcom is your equivalent to the USA's FCC per an Internet search. Ironically, enough the FCC utilizes a LOT of volunteers to do the independent investigations for them because they are very reliable and take their jobs extremely seriously. For example, there are local volunteer Amateur Radio monitors. If they see/hear abuse, they report the station's call-sign, date, time, etc. What these folks have in common is many years of being licensed, involvement in their communities, attending field days, etc. They basically warn someone and speak out about a problem, but if that does not fix it, they commonly fill out an FCC form where they make declarations as to what they witnessed or investigated. Typically, it is a small group that sign it. That is generally how this works in the USA. A letter then goes out warning of possible enforcement action, and any further complaint or follow-up for the same violation not resolved usually involves a fine, which is contestable and/or negotiable. The FCC is surprisingly fair as far as Government regulating bodies go.
Although it is hard to believe, a complaint of this nature would likely be investigated within 24 to 48 hours. You might be surprised how helpful Ofcom might be using tax dollars to help you.
I hope that helps.