It can be pronounced both Hew-ell or How-ell. I was told that the pronunciation varies in different parts of Wales which is why you can pronounce it in two different ways.
"Lily Rose was the eldest of the Ruggles family. Her great trial in life was her name, for she was a red-haired stoutish child and bore no resemblance to a lily of any kind or a rose either unless it was a cabbage one." The Family From One End Street, 1937