The nature of sewers is such that you cannot just roll up, drop down and examine a pipe. It must not be forgotten that the primary purpose of sewers is to move sewage after all. Anything that interferes with the sewage companies ability to do that is not going to be tolerated.
Quite apart from the problems of recruiting network engineers who are willing to work in a sewer environment. It is not your bog standard geek appointment after all.
At the end of the day, however, I suspect that financial considerations will win out. There can be no denying that the UK needs faster broadband, and it needs it delivered as quickly as possible.
Anything that can help deliver the promise of a 100Mbps broadband nation, without breaking the budget, has got to be taken seriously. You would have to be round the bend to think otherwise.