Low Flying Rule?
 

How was this flight in any way legal given that Headingley is in the middle of a huge built-up area?
https://www.theguardian.com/sport/20...future-is-safe

What makes you think it wasn't legal?

I seem to recall something about being able to be at such an altitude so as to glide clear in event of engine failure. Given that parks and roads in built up areas are not considered safe landing area and the fact he was towing a banner, he was far too low. Even the banner accidentally releasing would prove quite a hazard as the lead pole is usually substantial.
Even the 'fly spraying' that used to be over Arab cities required a twin aircraft

From the UK CAA Website.

It is difficult to know, just from that photo, if regulations were breached.

"(d) The land clear rule
An aircraft flying over a congested area of a city town or settlement shall not fly below such height as will permit, in the event of a power unit failure, the aircraft to land clear of the congested area."
There are exceptions for VFR routes. landing/takeoff, etc. But I can see no exceptions for banner towing!

At a guess it's no lower than 1500ft above ground. I don't know what the gliding range of a Cub is but I can see a suitable field 1.25nm to the west.

The banner would be released, that rule does not say the aircraft and banner.

If your as concerned as it seems Aneas, why not make a report?;

https://www.caa.co.uk/Our-work/Make-...-aviation-law/

Banner towing would be a commercial operation and the operator probably has a dispensation or Certificate of Waiver to do so.

You can break pretty much any rule as long as you ask permission and get approval to do so and have the paperwork to back it up.