Alta Adjustable Rear Control Arms:
Alta Mini Performance - Rear Control Arms
As for the front, I've the Ireland Engineering Adjustable Camber Plates fitted.
I'd lower the car only if I was able to adjust the rear camber, that's the one which changes lots. The front is ok as it is even lowering, you can fit the camber plates if you want to be able to work on your car handling, but it's not necessary if you just wish to lower the car. The rear, instead...well, the camber becomes quite negative so I'd not keep it as it becomes when lowering the car.