Well, there does exist a fixed pay scale for base salaries, but for senior academic grades in many occasions this simply defines the minimum salary, and the rest is negotiable (particularly if the academic has a good record of obtaining research grants).

Then again, there exist Business Schools, where due to the different funding model (most income comes from MBAs, as opposed to research grant overheads, and undergraduate tuition fees), full professors can easily make near-£200k in base salary.

Even with a salary of 200k per year, it's unlikely they will be able to afford a 2 or 3 bedroom flat in South Kensington. According to Zoopla/Foxtons, the average price is around 2M.

Senior professors negotiate their pay.

I don't know what you think that looks like in practice... but it's going to be very very modest at a British public university.