Two people earning 15k are £250 per month better than one person earning 30k, it's true.
With a total income of 2215 p/m though, it's not exactly dining at the ritz level. In the SE, that income would be breadline level for a family - let's say a 1 child one who aren't budgeting masterminds. Around half - at least - would disappear in rent for a modest family home at 'market rates'. But hey, let's call it 800 and assume they'll make do with a 1.5 bed shithole.
That leaves 1400 for everything else.
Run a car, circa £200 gone all in amortized over car's life.
Gas/leccy/council tax/water - not less than 300.
Food - let's say they are reasonably decent shoppers, and call it 250 p/m. Less than a tenner a day for the 3 of them.
We're down to 600 p/m left already.That needs to cover...
Clothes, cleaning, toiletries, some form of entertainment, child care etc. Hell even the occasional camping weekend.
All this before saving for a pension (best of luck with the tiny amount left), life insurance, dentistry etc.
Not to mention actual savings for emergencies, or god forbid, a deposit on a house (which would never be affordable anyway in the SE).
30k. The stuff of dreams are made off.