Transferred topic from "Ring circuits U.K. style."
I hope you enjoyed your trip to London. It's got a lot of history, but can be very confusing for visitors. It's also very congested these days. Anyway, your questions.....
In London and most other large towns distribution is underground by armored cables. You may see covers for inspection and joint chambers in the road every so often. Even in smaller places there has been something of a move toward underground services in recent years, and driving round England you'll certainly see less in the way of overhead lines than is usual in American towns.
Rural areas are still mostly overhead, but even here it's becoming increasingly common for the individual service cable to a home to be underground from pole to house.
For a given number of houses, we tend to use a smaller number of larger xfmrs than in America. Smaller xfmrs feeding a few houses in rural areas are sometimes pole mounted, but most are mounted on a concrete pad inside a purpose-built fenced and locked enclosure, so you wouldn't see them unless you went searching.
This also helps account for the 3-ph distribution used, as the same xfmrs often provide a 3-ph 4-w 240/415V service for commercial buildings.
Some of the London & other big-town sub-stations are totally enclosed in brick buildings. I'd guess that many of these could have been the old generator sites for the original DC systems which were installed in many places.
Supply to local xfmrs is usually 3-ph 3-w 11kV (line-to-line), which is also the service given to industrial users who have their own xfmr banks. Again, the 11kV is a mixture of overhead & underground depending upon the area and conditions.