Thats a good question, to which I dont know the precise answer.
Southampton and Bournemouth, Birmingham, City and Gatwick all have class D veils, and I think the veil is protected by Birmingham approach, City radar, Gatwick director and Solent radar. The flight guide will tell you and the notes bottom left of the CAA chart can be helpful, but beyond that how you would know from looking only at the map I dont know.
Calling xyz radar and you will not go far wrong. There are those who call them all sorts of things but mostly in anger when they refuse a clearance.