This cryptic message has been on display on the M25 for some days now. I've been ignoring it, as one does the "don't read distracting messages" message they (don't) put up. But I just googled it, and it turns out to be important.
Going over the QE2 bridge (always a pleasure) or through the Dartford Tunnel (I remember when the tunnel went both ways) used to cost £1.50, although it then went up to £2. Now it'll cost £2.50, but you can't pay at the barrier. They're abolishing the barrier. Instead, you have to prepay (or postpay) online.
Maybe this will save a minute or two as we cross. It should save TfL quite a lot in the cost of collecting the toll.
You can also sign up for a pre-paid account - if you do, then you can just forget the whole thing, it'll cost you £1.67 per crossing. That's what I've done, and I've registered three cars under one account.