I would dispute the research slightly, not only is CountryWide data probably not the best out there (they'll deal with higher end vs. huge non-"high street agent" market), but grouping all of London together will be massively skewed by the unbelievably high rental values in prime central london.
That's not to say London isn't expensive, and a big chunk of income goes towards it, but in terms of the actual percentages - I'd take it with a pinch of salt.
Most referencing companies will look for 2.5x rent -> gross salary. So if you earn 30k annual salary, you can afford 1k per month on rent. If you look at:
https://listentotaxman.com/30000?plan=0
You'll be taking home roughly £1,950. So that's roughly 50% of take home going on rent (54% if you're repaying student loans). Anything higher than that, and a referencing company won't pass you - so it seems odd CountryWide are suggesting their average is 57%.