Obviously the answer must depend on
a) disposable income, and
b) what else the pilot has to do in his/her life
Now, a) is the difference between income and "hard" outgoings and varies enormously. I know of men on £100k who haven't got anything spare, after sending 3 kids to private school, paying for the wife's credit card bills, paying the mortgage, paying the HP on the Mitsubishi Shogun which is necessary for the school run to any respectable private school.
If you are on £20k net and live with your mum, you will have loads of spare dosh; probably about £15k.
So knowing what the average PPL salary is wouldn't mean anything.
Average UK male is on about £22k gross and if living alone that isn't going to leave much for hobbies. The trick is to shack up with somebody else who is also working, avoid accumulating the usual UK symbols of copious consumption, and not have kids
Oh nearly forgot... avoid divorce!
Regarding b), that is the reason for why the UK GA scene is mostly a bunch of anoraks. Most people with some spare dosh tend to use it to enhance their social life first, and the younger they are the more deliberately they go about doing this. After they have fixed up their social life, found a partner etc, do they pursue other hobbies like flying.
The cost of flying also varies enormously. If you confine yourself to a local bimble on a sunny day, once a month, that's going to cost a lot less than IFR currency and 100-200hrs/year.