Is this a Hammerhead Twister 250cc Buggy?
I'm not very familiar with those. Buggy wiring is typically a lot different than quad wiring in the battery charging and lighting areas. The lights tend to run off the AC output of the low voltage high current winding of the stator rather than the regulated DC battery side (don't confuse this with the high voltage low current ignition power winding which is completely separate). Also the ignition systems on buggies are somtimes shut down by methods other than the kill switch, such as disconnecting the timing trigger signal, or removing the ground to the CDI.
To the best of my knowledge the hammerhead twisters do use only the kill switch connections for engine shutdown. I have a wiring diagram for a 2007 hammerhead 150cc buggy. And here is a problem: In an earlier post I said that the ignition systems of almost all quad engines run off an AC high voltage winding on the stator. Hammerhead buggies are one of the exceptions. Starting around 2007 (on the 150 cc engines at least) they started using special CDI's that ran on 12 volts DC instead of 80-250 volts AC off the stator. Before 2007 they used the standard ignition power winding on the stator and standard (easy to find) CDI modules. Whether this also applies to the 250cc engines I don't know.
Here is a place that knows a lot more about buggies than me:
If you ask around there I promise you will get lots of help.
The reason for starting at the kill switch connection (checking for shorts while the engine is stopped, and measuring AC and DC voltages here while cranking) is that this measurement point is right in the middle of the ignition system. High voltage here means that the high voltage power supply in the CDI is working, which eliminates the the stator and ignition power stator wiring. In this case we would next look at the trigger pulse, the CDI itself, the coil and the spark plug. Lack of voltage at the kill switch connection would direct the effort to the ignition power input wiring of the ignition power winding on the stator.
As far as the ignition coil being the common failure item - this is not my experience at all. Ninety percent of all problems I've experienced or read about involve bad connections in the wiring rather than failed parts. Chinese connectors are awful. Most of the time I read about people who double and triple check the connections only to find out later that they overlooked the one that was causing the problem.
As far as sources for parts: eBay has had everything I needed for my 150cc quad. Buggynews has several parts dealers that offer good free advice and sell parts - shipping to London though is likely a problem because of transportion costs and customs issues. I'd try eBay Europe first.