By "manual" do you mean no power assist?
If so, I have them on my MGB-disks in the front and drums in the back. They picked up power assist in 75, but honestly I don't notice a ton of difference in pedal feel. The ratio also changed so there's less pedal travel, but it's not really any easier to push.
Several other British cars of the era also had front disks without assist. The MGA Twin Cam and Deluxe(which is basically a Twin Cam with a standard 1600 engine) has 4 wheel disks without assist.
When you have a 2000 or 2500lb car, it's not a big deal to not have it. The first time I drove the MG, it definitely felt like it wouldn't stop, but I adjusted to the brakes pretty quickly and now don't really notice the lack of assist. In fact I get jumpy if push the pedal and it starts to feel soft(and I've driven a few other peoples cars that downright scared me at how soft their pedal was...)
When the factory added 2 more cylinders in cast iron(and 200 extra pounds forward of the front axle) along with a bunch of other stuff to support it in the MGC, they added boosters. The MGB GT V8 also had them-it's actually not much heavier than a 4 cylinder MGB GT but has bigger brakes.
I owned a 74 Marina for a little while back a few years ago(and had my ups and downs with that car). The ugly US spec one weighs about 3000lbs, or at least my auto transmission one did by the door plate. I never did a ton with the brakes(other than freeing up stuck rear drums) but it definitely has front disks. I forget what they cross reference to, although I think the brake set-up on one is pretty much bolt-on to a Morris Minor. I don't recall ever having to mess with a booster, and looking at my photos of the engine bay I don't see any obvious one nor do I see any obvious vacuum lines that would run to one. The few times I drove, the brakes were the one thing that worked perfectly fine and I don't recall pedal effort being especially high.
EDIT: Never mind on the Marina-it does have a booster. I don't know how I missed the vac line on mine, and I looked at a few other photos of ones that show the booster.
