Where you live makes a big difference to how far your money will go, but because of the disparity caused by this it also has a big impact on social mobility. The sort of property you live in also makes a big difference to how well off you feel and how much disposable income you have.
Also these figures are before tax, as a rough guide for incomes after tax and NI:
£16,000 => £700 income tax, £884 NI, leaving £14,416
£40,000 => £5,500 income tax, £3,764, leaving £30,736
This is based on an individual and so how the household income is made up has an impact, two earners on £16k almost have less than £1k difference after tax as a household with a single earner on £40k - although for couples where there is only a single earner they can in theory make use of the £1,250 marriage allowance offset, but for single parents and unmarried couples that obviously isn't available. I'm not sure how many people would know about this or claim it either, I only do as I'm self employed. It also doesn't take into account things like pension contributions which would usually be based on a percentage of income.
After income there will be certain regular costs, the biggest of which will be accommodation. Location and whether you rent or pay a mortgage and when you bought will have a huge impact.
As a guide the first house I bought back in 2004 was in Reading and a 3 bed terrace - it was on the market for £166k and the mortgage then was around £700 a month and it was among the cheapest of properties in the area at the time. Now to by a similar property there would cost about £250k and the mortgage would be about £1,000.
A similar 3 bedroom terrace in Lancashire to buy now would be about £150k and the mortgage of about £540 a month. And I remember from looking in 2004, as we'd considered moving up there, a 3 bedroom terrace used to be around £90k but can't remember what the mortgage would have been.
I think in both cases for equivalent rents you could expect that to be the monthly mortgage plus 20%.
Travel can also be a big factor, commuting into London from Reading daily (with underground) is £5,468 for an annual season ticket at current prices, I believe it used to be a couple of thousand cheaper when I first started doing the commute. Buying and running a second hand car may not that much cheaper by the time you factor in fuel, insurance, servicing although that will be influenced by how long you keep it. And again depending on where you live and work both may be necessary.
So if you got on the property ladder a long time ago, can walk to work, and live far from London (typically) the same income may well go further than if you're young or rent, live in a more expensive area, or have to go further afield for work.
Also it is worth noting there is a difference between mean and median - so when you say average, if you take that as the mean that average is skewed upwards by the very wealthy, whereas median is essentially the middle number so 50% are above and 50% are below.
That means even by the most generous measure 50% of people survive on less than £30k household income sometimes significantly so. And that can have a significant impact nationwide as it can mean a reliance on food banks, choosing between eating and heating, and leave people stuck living with family, where they can afford, or where they are given accommodation sometimes unable to live near family and not having the option of moving for work or better paid work - and again that creates a big social mobility problem and will tend to disproportionately affect younger people, or those renting, and in low paid jobs.
Perhaps next time you're in the supermarket you could buy a couple of extra tins of tomatoes or a pace of rice or two and leave it in the boxes they often have for foodbank donations.