"Almost everyone" is basically almost everyone you know. It's difficult to find data on the market share of pay as you go plans in the UK but I'm sure it's a significant portion of the market. These plans typically charge for individual text messages unless you get a special plan or meet some criteria (e.g. top up at least £10 a month).
It's huge. The most recent OFCOM market report reckons just over half of all mobile customers in the UK are on PAYGO. Don't under-estimate the UK pre-pay (PAYGO) market. It's quite easy to, because most people reading this are probably on contract themselves.
Not only is pre-pay half the market in the UK, it's by far the hardest to retain. Pre-pay customers aren't loyal (partly because they're not locked in to contracts, partly because they tend to be price conscious), and because they skew young they tend to be the customers more likely to both a) use SMS, and b) switch to things like WhatsApp.
I find that really surprising because phone service is now just so damn cheap in the UK. After a cashback offer, I'm effectively paying £5 a month for more minutes, texts and data than I can hope to consume. I guess my needs are relatively modest, but so (I imagine) are most non-business customers'.
Pay as you go plans also typically charge a fairly significant amount for mobile data too, though. In fact, on giffgaff (a PAYG-only operator which seems to be insanely popular with young people here in the UK), the most sensible way of paying for mobile data comes bundled with unlimited texts anyway.