Theres nothing illigal about a hydraulic handbrake in a road car. I currently have a 3 door Sierra and a 5 door Sierra, both with Hydraulic handbrakes and MOT's so if you want one you should be OK
What you do have to have though is a mechanical parking brake. Its so if you lose hydraulic pressure, say a leak or something overnight, then the cars not going to roll down a hill.
I retain the normal handbrake mechanism in the Sierras and mount vertical hydo's in them. Having 2 handbrakes may not be as clean looking as you want though for a kit / race / custom car though. There are available, from places like Rally Design and Demon Tweeks hydro handbrakes with optional cable mechanisms. Bank on about an extra £60 on top of the hydro for the kit.
Hope this helps. It may be different for you new tests for custom builds but thats what I know from MOT's and road experience