Following up on Bob's suggestion, as you won't be able to get Rockler stuff in the UK, the jig itself is easy enough to make. I made mine with a sample rectangle of Lexan that I scrounged from a shop that sold the stuff.
You can fix a fence to it with csk screws. You can then trace the position of the big hole and screw holes on to it.
Now then,there are a couple of possible sizes for the main hole.
If you use a short bit with a top bearing on it, the hole can be the same size as the one you want to cut out for the hinge.
However, Rockler suggest doing it with a guide bush, which gives you more flexibility on bit length. If you do it their way, the main hole will need to be larger than the one you intend to cut for the hinge as you need to allow for the bit being within the guide bush and not bearing directly on the jig. The size will thus depend on the guide bushes and bit sizes you have available.
I liked their idea of using the screw hole guides to clamp the jig in place.
On the other hand, and I know we are a router forum, you'd be much quicker getting hold of the right sized Forstner bit and doing them in the drill press.