Hey, is it raining during the London commute? Do you have a FENDER on that OTT racing bicycle of yours? No? Isn't that INCREDIBLY THOUGHTLESS AND SELFISH OF YOU?
And besides, the wet stripe up the crack of your arse makes us all wonder if you're incontinent (not nice first thing in the morning, believe me), so even if you can't be asked to protect your fellow cyclists from your chicken-tail-of-road-grime, then at least have some modesty!
And! Don't bother with one of those silly flat horizontal fenders. Even *if* that will keep your own arse dry, it doesn't do the rest of us any good. They, frankly, shouldn't be sold. Get an *effective* full-covereage fender if you're going to commute in the wet.