...but it's not due to all that concrete and steel. And in fact 1/5th of Milton Keynes is open access parkland so it's not the concrete jungle the urban myth makes out (that is if you decide to step out of the car, and look at the place beyond CMK.)
There are two reasons:
1. MK sits in a number of shallow but there none-the-less valleys, this doesn't help mobile signals on some networks but I have never had an issue (TMobile and Orange)
2. Most of the new town was laid out with aluminium telephone cabling, not copper because it was cheaper. And aluminium is terrible.
In theory both of these problems can be overcome.
That said, it's a pretty inexcusable situation.