TFL is a "Statutory Corporation" which is funded both by grants from local government and commercial income, the majority from rail+bus fares but also from activities such as advertising. There is a significant quantity of advertising placements provided by TFL, in the stations, on buses, bus stops, and as it seems on their website.

TFL was almost bankrupted by Covid, I for one will give them a pass on the ugly advertising on their site.

However, looking at [0] and [1], advertising only covers about 1% of their budget. That banner ad will be tiny fragment of that.

0: https://tfl.gov.uk/corporate/about-tfl/how-we-work/how-we-ar...

1: https://www.statista.com/statistics/898278/transport-for-lon...