Hi Mike, I think a franchise might work but they would have to specialize in one or two makes for cost, training and quality reasons.
How about this idea:-
The tool for the job would be autologic as this is one of the few portable tools (other than dealer) where you could start the job and be reasonably confident you can finish it!
The tool is relatively easy to use (unlike dealer tools) and if you are having difficulties help is just a phone call away (again, unlike dealer tools).
I think you would need a fairly large catchment area though (major city?) to generate enough custom to support several franchisees.
The franchisees could be sold vehicle makes on a first come first choice basis and as mentioned earlier limited to no more than two makes.
Having said all that the tool is only as good as the guy using it and fixing modern diagnostic faults at someones door is not as easy as it sounds.
Amongst other things you have to deal with is the British weather, the customer breathing down your neck with his stopwatch in his hand!
I suppose you could decide to do trade work only, use their premises, ramps, electricity etc.
The added bonus to that one is they would deal with the customer but one of the downsides is that you might end up with all the b*****d jobs as they will do the simple (quick) jobs themselves!
It's possible I suppose but you would have sit down with a financial expert and cost it all out, then you would see if it's a viable proposition.

Interesting one Mike.