Surely we have all had this issue on a bit of a long tube journey - really needing the loo. There are benefits to knowing where the loos are on the tubes, for example if a friend of mine some years ago had known which stop on her morning route had a toilet near it she wouldn't have had to be sick three times on a tube carriage.
Luckily some clever person has made a tube map with all the toilets on it. Surely a blessing as we all know these are not easy to find.
An interesting fact seems to be that toilets are favoured on the outskirts of London rather than the inner circle, probably because by the time you get further out you'll be busting more and more.