Looking more closely on glassdoor: it's unlikely to be true that Imperial professor salaries average £80k and University College London salaries average £101k, and these figures are more likely biased by small, self-selecting samples.

[Updated to note: Imperial College quotes a fixed minimum salary of £79,080 for professors in 2019 with performance-related increases - so the Glassdoor figures quoting lower than that cannot be right]

The home.co.uk site quotes the median price of property listings is £600k (is that close to £1m? ok sure). It also identifies that houses priced higher than that are listed for significantly longer, i.e. they count with more weight in this stat despite not selling. Therefore, I understand the median sale price to be lower.