What is a "longstop"? (British, I think)

I believe that "longstop" is referring to the cricket position, and that makes sense in context: the job of the longstop is to act as backup for the wicket keeper.

If everything goes according to plan, they will never have to do anything - the wicket keeper will stop the ball. But, sometimes, due to a mistake by the wicket keeper, or an unexpected bounce, etc, the ball will get past the wicket keeper and the longstop will step in to stop the ball instead.

In this sense, the longstop position is a form of insurance - like a breakdown specialist, and an insurance company (the other examples) - you only need them when things go wrong, or not as planned, but when you need them you are glad that they're there.

The writer is saying that God might be like this - that He only intervenes in emergencies, perhaps to save someone from dying in an accident, or when someone is very depressed, etc.