Improve OS data prep

[dabreegster]

• Change what the speed layer contains, based on feedback. (All time periods, average of the direction?)
• Include roadclassification or similar and use it to simplify low-zoom by gradually exposing smaller roads
• Filter out Scotland and Wales

Perf:

• Flatgeobuf as intermediate output?
• Including less features at low zoom?
• Per @stuartlynn, using line-delimited GJ and https://github.com/mapbox/tippecanoe#parallel-processing-of-input