It's a union of three kingdoms, or 2-and-a-bit kingdoms:

   England and Wales (927)
 + Scotland (1707)
 + Ireland (1802)
 - most of Ireland (1922), leaving only Northern Ireland

The United Kingdom of Great Britain and Northern Ireland would probably change its name if Scotland seceded, but perhaps to the United Kingdom of England and Wales and Northern Ireland. The official ISO code (GB) would change (EI? EW?) but the reserved code (UK) need not.