Almost everyone in the UK has an unlimited or close to unlimited texting plan (3000+ texts). Given that everyone's paying for it anyway, the actual revenue gained by someone sending a text is 0. In fact, because a text will use some tiny amount of infrastructure, they'll potentially lose more money the more texts people send.
Most people I know who use whatsapp use it more like an IM client for group messages rather than for one-to-one messaging. As everyone has more or less unlimited texting available to them, nobody uses whatsapp for financial reasons because there's no savings made.
This will only be an issue once people start to realise that they're not using as many texts and demand to pay less. I can't see that happening in the UK.
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.
No, the problem is the general rise of IP/push messaging across all sectors. Carriers do make significant amounts of money from direct connection billing and bulk SMS. As more brands develop mobile presences the need to contact customers through SMS versus alternative methods will decrease. Push messaging is considerably cheaper, and from the end user's perspective often less annoying and more convenient.
Fortunately (for the carriers), most operate globally, and certain developing markets offer vast SMS revenue potential, more than the West ever did. India alone is a massive user of SMS, considerably more so than the US (partly due to the lack of regulation: a lot of those SMS messages being sent in India are spam).