I can't speak for British conditions at all, but in the US the cost of such insurance is prohibitively expensive. If you even consider the possibility of a medical grounding, then you'll never even start flying. What is merely a minor inconvenience to most people can ground a pilot for life, or at least several years. That's why employers must treat pilots differently from other employees, who are not limited by the requirements of an annual medical exam. If you don't have lots of available sick leave and vacation time, you're going to be in financial trouble eventually, unless you're fantastically lucky.