The restrictions are due to MOD (Ministry of Defense, military) use. There are also some secondary business users in the spectrum. If you would look at the primary allocations in the UK, you might be surprised by the amount of spectrum allocated to the MOD. Ofcom maintains spectrum allocation data on this topic:
UK65 - By special arrangement civil Land Mobile services may operate
within sub-bands 425.0-429.0 MHz, 431.0-432.0 MHz, 440.0-443.5 MHz,
and 445.5-449.5 MHz in agreed areas and on a non-interference basis to
military services. Further information can be found in the document
"Frequency sharing arrangements between civil and military services".
(I also used to live in the UK)