I used to do it - IOM, cross channel, but that was when I was young and immortal. Later, I had a re-think and decided SEP over water isn't an acceptable risk. Why? Because one dictum I have stuck to in 30 odd years flying is 'always have an out'.
If the engine fails over land, your 'out' is a forced landing which should have a pretty good chance of turning out OK as long as you don't fly over miles of forest or somewhere else unlandable.
Over water, especially around UK, if the engine fails you are probably going to die. Especially in a Chipmunk (fixed gear, no space for a dingy etc). Even in a retractable with a dingy your chances are a bit better, but not that much.
So will the engine fail? Almost certainly not. But how lucky do you feel, punk? A piston engine is a mass of reciprocating and rotating parts all eager to part company with each other, with a thin film of pressurised oil preventing the whole thing seizing up. That they work at all is a marvel. I've had them fail and I know folk who've had quite a few fail. It does happen.
And when it does.... where's your 'out'?