Been away canoe camping for a few days in hot weather? got a long way to drive home? Did you know that most motorway services in the uk have showers for long distance drivers and anyone can use them. most are free, a few charge a nominal fee.
most services have a time limit on free parking - usually about two hours but may be less - check the signs in the car park
I found a list of uk motorway services, if you select one, then look under "facilities - general" it will tell you if they have showers there. some may be cleaner than others but then, if you've just got out of an canal or uban river...
not near a motorway? most leisure centers will let you use their showers for a couple of quid.