£500 for Bankside is pretty ridiculous in my view. Unless it’s a very specific date causing the rate, I’d look elsewhere - including outside of Marriott if they’re all so highly priced. Since Covid I have noticed Marriott prices have gone up much faster/ higher compared to most other hotels.

Pricing every day is different, but unless it’s a specific peak day, you should be able to do better for £300-350ish. There are usually 2-3 weekends or dates a year where it’s every hotel which is nuts.