Chris wrote:

Unlike FM where all the National stations broadcast on different
frequencies, the National Multiplexes on DAB (which includes all the
national BBC stations) are on fixed frequencies so there is no need for the
DAB radio to retune - it just needs to be in range of a transmitter.

There's one exception to this. The Digital One mux (that carries the
national commercial radio services) uses a different frequency in
Scotland, and doesn't broadcast at all in Northern Ireland. However
some of the D1 stations are available on the regional NI Mux