Specialize for England

#363 started https://a-b-street.github.io/ltn/england.html.

- [ ] Equivalent of settlement boundaries, to prune out rural areas
- [ ] Some contextual layers: bus routes, population, railway stations, stats19
- [ ] All the same prioritization metrics as Scotland (equivalent data exists in England too)
- [x] Demand data (need to decide the source)
- [ ] Demand data that's actually interesting -- either 2021 has tiny counts or the OA-level detail is inappropriate

Look at https://shapeatlas.net/ for more detailed IMD data

Start by examining the existing Scotland implementation in the codebase to understand the data processing and visualization pipeline. Look for files handling settlement boundaries, contextual layers (bus routes, population, etc.), and prioritization metrics. Investigate the England-specific data sources mentioned, such as stats19 and shapeatlas.net for IMD data. The goal is to replicate Scotland's features for England, integrating equivalent datasets and ensuring the web interface displays them correctly.

Written by the indexing model from the issue text.