best whitening shampoo I know (and I've got 2 white tails) is Boots own brand economy - about 36p for a bottle - it really makes white hair silvery - amazing!
I will be washing my mostly white Appaloosa tomorrow with fairy liquid, he should be sparkling clean ready for him to go and instantly find the muddiest place to role in!
Another vote for Daz. I make up a strong solution of Daz in warm water and wash end of tail in it, wrap a bit of cling film over while you spruce rest of pony then rinse off = sparkling white tail. I usually rub a bit of conditioner for blond hair in, then rinse off - seems to protect the white from the wee for several days longer (she always wees on bottom of tail).