Ambassador parking brake improvement?

Possible, yes, easy, no. The brakes either have to be from the same axle, either AMC 15 or AMC 20.  Remember, Ivor is in England, so hard to find another AMC donor. Heck, it's not easy to find one over here any more!

Theoretically, they can come from any axle -- any make or even year. The backing plates and all would be needed. Bolt pattern isn't even a big issue -- drums can be drilled for a different pattern. The critical measurement is the offset from the axle flange where the backing plate bolts on and the mounting surface of the hub. Even if that's the same, there may be an offset in the drum itself -- that's how AMC made 2" and 2.5" wide drums fir the same axle. You may need the park brake cables from the donor axle as well, but that's the easy part. Might need a longer or shorter cable from the handle/pedal to the axle cables, or a custom balance bar, but if you get around everything else that would be easy! Here in the US we could probably adapt Ford truck/big rear drive car brakes easily, they are usually Bendix, similar to AMC Bendix brakes. Of course you may need to change to a different size wheel cylinder or use a proportioning valve in the rear brake line.

If the rear brakes are sized correctly a proportioning valve isn't needed, even with disc front/drum rear. Such valves were used in the 60s and 70s, but by the 80s brakes were balanced by size. Few modern cars use them, but some do due to extreme weight differences front to back or because the engineers wanted to use an existing part -- or economics dictated they had to. It's easier to dial in an adjustable valve than to get the size just right, especially when modifying and using existing parts.  You could use a proportioning valve to dial down the rear brakes and keep the same front brakes -- no upgrading required.

Well, you mirrored the photo...flip it back!!

Ok, I know it's a right hand drive export car...  The Belgian built "Renault Rambler" was always a Classic with Ambo trim IIRC (all except the things that say Ambo...). You do say it's a Canadian built car... so export from Brampton. Anyway, the US cars have a foot pedal operated park brake, it was easier/cheaper to use the "umbrella handle" on exports -- not high enough numbers to make a foot pedal assembly I guess.

Another thought... if you have an issue with getting enough power from the hand assembly, try finding a foot operated assembly from some other car. You'd have to find one that easily comes off and is pretty self contained, then figure out how to reinforce the Rambler to mount it though. So if increasing leverage will do the trick... At least this is a "plan B".