Bear in mind that the UK has a “national speed limit” of 60mph for much of the countryside. This is very much a limit, a maximum, and you’re expected to drive to the conditions of the road. If it’s perfect weather conditions and twisting roads not wide enough for 2 cars, you shouldn’t be driving at the speed limit.

Having driven in the US and UK, this is a significant difference between the two. In the UK, you might sometimes drive 30 under on a road that is nominally 60 mph. In the US, that road would have a specific posted speed limit that is safe to drive. US roads are also more consistently designed for constant speed or have additional advisory speed limits for curves. You can nearly always drive as fast as the number on the sign unless there is some additional hazard.

Absolutely. The legal speed limit is 60 in the country - on any road not marked with a lower speed limit. This means that legally, you can drive at 60mph down a twisty single track road with 1.5m earth and rock banks topped with hedges.

You would be an irresponsible nutter with a death wish to try through! And if you crashed, "I was driving at / under the speed limit" wouldn't wash - you would be charged with Driving without Due Care and Attention, or Dangerous Driving depending on the consequences of the crash.