Well, I don't think everyone does agree. I, for one, don't think it is essential.
Think about it. Would you expect to pay the same price for a bus into town and to get to Edinburgh? Hardly. Why should mail be any different? We understand we need to pay more to send a letter to, say, Hong Kong - why shouldn't it cost more to send it somewhere far distant and expensive to reach in the British Isles.
Yes, you might say, but what about the poor people who live at the furthest reaches of the UK? What indeed? They have every right to live there. But I'm not sure they have a right to expect me to subsidise their postal service every time I send something just down the road to Salisbury.
I think part of the problem with the universal post system is the lack of creativity applied to getting around the practical difficulties of doing anything different. At first sight you might imagine it would be a nightmare, having to work out how far it is to your destination. And it would be if you did this (although the railway manages to operate a fares system that copes - and so could the Post Office). But here are two easy-to-implement alternatives:
- Simple banding. Anything to the same postcode letters is local, anything in the same country is country, anything to a different country within the UK is crossborder. Three bands, automatically detectable from the postcode. Easy peasy.
- Receiver pays. This is the more interesting idea. Still have a universal price to send a letter anywhere in the UK. But if you live more than a certain distance from the nearest sorting office, you pay a premium to receive your post (or go and collect it for free). This would be fiddly to do on a per-item basis, but could be simply implemented monthly or annually through the council tax collection.
It's not perfect - it's top of the head, 5 minutes thought stuff, I admit it. There would be extra complications. But the fact is that the idea of having a single price for a letter to anywhere in the UK is not sacrosanct, it is not written into the constitution (hardly surprising when we don't have a written constitution), and it's not necessarily the best way to go about things. It is at least worth thinking about the alternatives, rather than taking the usual stick-our-heads-in-the-sand approach.
Image from Wikipedia