the majority taking the survey are male, middle aged, have high incomes
I’m not sure you can make that second leap from the survey.
Given the age range is average 41-50 and that many households are going to have at least two people earning (possibly more, especially in lower income households where multiple renters is common) it doesn’t need much to tick into the “higher” brackets (unless the distribution has changed most were in the up to 100k bracket).
2 forty ish year olds on median salary aren’t far off that bracket, add in a teenager with a Saturday job and you’re into that one.
Also the gender pay gap is wider at higher age groups so “male and 40+” is going to heavily skew the average wage upwards too (though likely in the same households one of the other earners will be female in the same broad age group so household income tends to be closer to the median)
4 people on 39 hours at minimum wage puts you in the same bracket and isn’t unlikely if those 4 people want to “make rent” in a lot of places.
Source:
Ages 16-17: £10,910
Ages 18-21: £17,284
Ages 22-29: £24,600
Ages 30-39: £30,865
Ages 40-49: £33,477
Ages 50-59: £31,358
Ages 60+: £27,508
From https://occaminvesting.co.uk/average-uk-salary-by-age/