I use the dog brush (why not? ) to brush off any dry mud from round the bottom of trousers etc.
We do the same but not with a dog brush as we don't have one (a dog that is). Let the mud dry brush off what you can and then let them go and get dirty again
If we are camping for more than a week, I usually google launderettes in the area and we visit half way through the stay. That way I don't need to pack loads of clothes as space is precious in the car.
Loads of socks and undies, trackies for son and loads of leggings for daughter. One set of "good" clothes for going out and overall essential is Wellies! I have used laundry facilities but only when away for fortnight or to save washing when I get home. In Belgium last year I met some lovely Belgians waiting on the machine to finish, we even opened a bottle of wine together so chores weren't that bad
We use camp site washing machines but generally for washing cloth nappies in the middle of a holiday when staying for more than a weekend. If it was just us grown ups and no nappies or potty training toddlers we would not have not needed one.
I wouldnt bother trying to wash anything unless there is a tumble drier handy - by the time its dry it will probably be time to come home! Especially with the great British Weather.
I always set off with good intentions of keeping the kids looking clean and tidy then it all goes down hill. Somehow it doesnt seem to matter when you are camping. I remember stopping off at a service station on the way home once and having a good look at the kids in their grubby shorts and t-shirts then hoping I didnt see anyone I knew!!!
If after all these comments you still want to do laundry it may be easiest to contact a close by 'holiday park' and ask if you can buy day passes. This is our plan for when we go to Cornwall, we will drive there, let the children take advantage of the swimming pool then send them on to the play area with hubby while I get the laundry done. You may find they are open to the public and you can just pop on to do your laundry without a day pass, I'll be phoning closer to the time to find out.