Approx. £800 – £1,250 per week in most London areas

https://londonbusadvertising.com/

Nationwide from £195+VAT.

https://focusmediaoutdoor.com/bus-stop-advertising/

Now London would be a lot higher than the 2nd quote but between the two it's easy for someone to afford it if they were motivated without being a billionaire.

Most likely someone printed it out and replaced the advert that was there with it. The tools to do so are very cheap, I hear. https://brandalism.ch/take-action/

This community is for subvertisements, "The use of parodies of corporate and political advertisements in order to make an ironic statement."