Legal Requirements for Braking systems on Trikes/Recumbent Trikes

Hi

Not such a regular here these days but still lurking :-)

Anyway need some advice - currently looking at getting back on the road in
one of these
http://www.bike-on.com/newhandcycles/XLTPro.htm

but am investigating being able to swap wheels between 650c and 24" wheels
with fat tyres for those sustrans trails with the kids days.

In theory the only problem with this is the ability to match the brakes up.
These types of bikes usually have two brakes both on the front wheel to
comply (presumably) with rules on numbers of braking systems. Now I know a
bike has to have two braking systems to be road legal but does anyone know
if the same rules apply to a trike ? I'm proposing to fit a cable operated
disc brake to the front wheel but need to decide how to work the second
brake.

Any Ideas ?

Russ