Never heard of these being provided in UK on NHS. Always worth asking as protocols do vary between areas/hospitals. At my local hospital they don't even offer adhesive protheses so I dread to think what they'd say if I asked for a customised one!
The type that I wear (inside a pocketed bra) has a squidgy section on the back so effectively moulds itself to my chest wall anyway. What's the point of customised one if it's being worn inside a pocketed bra unless one's chest wall is very uneven? Or perhaps customised prostheses are adhesive? Excus my ignorance. There's such a range of sizes and 'droopiness' that I've never struggled getting a match to my remaining breast either.