IMHO thunderstorms in the UK are usually not too bad (there are exceptions!). They are not things to be disregarded but they are not instant death if you get within 5 miles of them.
Even if you end up inside one if you keep your head, keep the right way up, fly below maneuvering speed etc you should be ok. As I understand it structural failures tend to happen when an aircraft (ie pilot) gets upset, exceeds VNE and pulls up too sharply when falling out of the bottom.
In a certificated, bonded airframe lighting strikes are usually not a great problem.
If you look at reports of record breaking flights in Africa the pilots flew through the much worse African storms without disintegrating (though I expect not something they would want to do very often).