Good Job on the troubleshoot!
First I'd be concerned about the Coil Secondary reading, It Should read between 6,000 and 30,000 Ohms, If your not getting a reading, you have a problem.
Are you sure you didn't forget to RESET your DVOM to R X 10K or auto range when you changed sides of the coil? If you forgot, It would read in error. If your Sure you had it right,
I Would bet the Carbon element has broken down inside the coil housing, and allowing just enough spark to fire at load loads (Idle)
Pull the Coil from the cap, and measure between the Red Wire and the spot where the Carbon element touches the coil..
If you don't get a reading, replace the coil...If you do get a reading, replace the element and the rubber gasket in the cap (you may need to buy a new cap)
As far as the module goes, no real practical way to test it yourself, If the Auto Stores in the UK don't offer this service, the only other recourse is to just replace the module..
If you do this, and no change is present, than put the old module in the NEW box and toss it in the glove box, never know when you might need it on the road..100 KM from nowhere.
If it solves the problem, Chuck it in the Round file..What ever you decide to do, If you remove and replace the module use heatsink compound! or it will soon die.
As Far as the compression goes, If you can do that , It can show the state of the timing chain..If the cylinders are low, like around 50 or 75 Lbs..your chain has jumped..This can happen anytime the car is over 100,000 miles..
The other way, is line up the Timing marks, pull the left valve cover, watch and VERIFY both intake and exhaust are Dead Closed, The Piston is at TDC, Compression..If you can't coordinate this, the chain has jumped..
If you have that, Then pull the cap, the rotor should be DEAD on #1 Plug tower..If not, or off a bit in either direction, Pull and inspect the gear and it's shear pin, be sure it's good, reset the dizzy, for dead #1 Plug tower, and try it again..
Seems to be narrowed Down to those three items..