In any case, "According to the Organisation for Economic Co-operation and Development, middle class is someone who earns 75 per cent to 200 per cent of the median national income."[0] and, as you say, the median "Equivalised household disposable income of individuals" is £29,900 according to the ONS.[1]
I'm not sure if "disposable" here means that the OECD definition doesn't apply, but I agree that the article would have been less misleading if it had used a less subjective term, and a less extreme definition.
[0] https://inews.co.uk/news/politics/budget/middle-income-what-...
[1] https://www.ons.gov.uk/peoplepopulationandcommunity/personal...
I think it's probably very contextual. For example, The Telegraph reported that 'Seven out of 10 people view themselves as [middle class]'[0].
That's probably a result both of lower wage people feeling aspirational (and associating "working class" with manual labour) and many people with traditionally upper class traits feeling embarrassed to publicly suggest they are superior.
As the BBC notes, 'David Cameron - educated at Eton - has described himself and his wife - the daughter of a baronet - as part of the "sharp-elbowed middle classes".'[1]
[0] https://www.telegraph.co.uk/finance/personalfinance/8393834/...
Also, the "Tax Payers Alliance" is an opaquely-financed group with zero membership, which has existed for decades for the sole purpose of getting cited as a 'low tax pressure group' when newspapers feel the need to get 'the other point of view' for some story. Being an astroturf group obviously doesn't make it wrong in any given case, but it has neither a reputation for honesty, nor for actually being anti-tax (it's very loud about taxes mostly paid by the rich; much more quiet about VAT for example).