Ah, I think I understand. LaceSensor, the second page (this one) says "all flights are one-way inc. taxes", yes? But then it says "from" and gives a price. I suspect you'll find that some very unpopular dates really do cost £4.99 after taxes, which in this case can only be explained if Jet2 are paying the taxes themselves and letting you fly at a loss[*]. However, the vast majority of the prices accrue tax. Hence when you click through to the actual reservation system you suddenly see £50 or so as the typical sort of price.

It's somewhat weaselish, I agree, but it's no worse than most firms do. I'd also counterbalance all this by pointing out Jet2 are easily your cheapest way of getting to your destination, taxes or no taxes.

[*]there are a number of complicated reasons that this would make sense. For example, airlines usually sign agreements with airports that they will fly X routes per year into it, which the airports in turn can use to sell commercial shop space inside the airport to businesses on the grounds there will likely be Y passengers per year passing through it. This can result in craziness like airlines flying empty planes or flying ringers into airports.