HE might be best known for his high range, but Warren Mok will only be taking to the skies this month if he gets a sponsor to buy him a ticket to London. Hong Kong's most famous, indeed only, tenor has been asked to take the principal role of Eduardo in Verdi's Un Giorno di Regno at the Dorset Festival, with rehearsals starting on Sunday. Trouble is, the festival people don't have the money to fly him there. ''When they invited me they thought I'd be in Berlin,'' said Warren, who is taking a holiday from the Berlin Opera company, to prepare La Traviata in Hong Kong next month. Warren told us he first decided to be a singer when he was a boy watching La Boheme in Hawaii. Now events have gone full circle, and he is planning to travel to Hawaii soon to take the part of Rudolfo in, of course, La Boheme.