Returns every valid UK boundary type code mapped to its human-readable label.
Call this before using any tool that accepts a `boundary_type` or
`boundary_types` argument so you know which codes are legal. Passing an
unlisted code to another tool raises a ValueError.
Boundary type codes are stable Ordnance Survey identifiers. Common ones:
- "CTY" → County
- "LBO" → London Borough
- "UTA" → Unitary Authority
- "MTD" → Metropolitan District
- "DIS" → District
- "DIW" → District Ward
- "CCTY" → Ceremonial County
- "HCTY" → Historic County
- "WMC" → Westminster Parliamentary Constituency
- "GLC" → Greater London Constituency
- "SWC" → Scotland/Wales Constituency
- "PAR" → Parish
- "CED" → County Electoral Division
Returns:
Dict mapping code → label for all supported boundary types,
e.g. {"CTY": "County", "LBO": "London Borough", ...}
Connector