Poor idle is unlikely to be the coil, the voltage demand at idle is very low
You will not be able to measure voltage with a voltmeter as the period that the voltage is present is very short
The voltage is not fixed by the coil, it depends on a number of variables including temperature and plug gap. The voltage across the plug will build until it is high enough to either jump the gap or jump elsewhere.
A resistance check is only likely to tell you if the coil is completely dead. if it is breaking down then you aren't likely to be able to tell, primary should be about 4.5 Ohms, secondary probably about 10 KOhms, I can measure one if you want.
UK 410s definately use points, check the gap and condition. Other possible problems are the plugs themselves, plug leads, dizzy cap and condensor. As far as I can tell from the manual there are two types of 410 dizzy, vacuum advance and mechanical advance, I would have expected both but there you go. You could have a sloppy advance mechanism.
Alternatively you could have a mechanical problem, its always worth giving it a compression check.