This was discussed recently on the Ausmini forum where a member asked about removing the bypass hose and thermostat when installing a Thermatic fan in the wheel well. I assume it wasn't you, Drakman?
The consensus was that the Mini (and Moke) setup is different to most other vehicles where the radiator is mounted behind the grille and hence subject to a strong cooling airflow when driving, so the original cooling system should be retained as is.
Here's some straight copy and pastes from that discussion.
I always thought the original fan remained in place so the electric fan only acted as a booster rather than a complete replacement. Instead of the electric motor running continuously it only comes on when the temperature reaches a certain level, perhaps in stop-start traffic or on a very hot day when the standard set-up can't cope. A well-maintained standard cooling arrangement should usually be adequate in most conditions.
Even on a coolish day, without a fan you will only get about 15km before the engine starts to overheat... and that's cruising at a fairly steady 80 kph - which are probably the most benign conditions the engine will run under... Maybe should have tried at 100 but I think the conditions would be worse...
I think radiator location is a major part of it.
With a frontal radiator, it sees 100% (for simplicity sake) of the airflow through the grill, after which the air exits below the engine, and through both wheel wells (let's say 1/3 each).
Mounting the radiator in the wheel wheel means it only gets 1/3 of the airflow coming through the grill.