There are thunderstorms and then there are thunderstorms.
Exactly. Many thunderstorms in UK are showers with lightning. Avoid but don't panic. As I understand it most in flight breakups occur when the aircraft (ie pilot) loses control in turbulence, falls out of the bottom in a fast spiral dive, maybe beyond VNE and then levels out and pulls up too sharply. I suspect that if you end up inside the average UK storm if you keep your head and keep the right way up you will probably be ok. (I have not tried this often enough to be sure).
If one reads accounts of light aircraft record attempts in Africa the pilots do punch through lines of storms much more powerful than the average British ones, trying to avoid the worst bits. They often have a very rough ride but (usually) don't die