Who was first british athlete to hold world record for javelin?

Fatima Whitbread.

Whitbread set the women's world record with a throw of 77.44 meters (254 feet, 3/4 inch) at the 1986 European Championships in Stuttgart.

One other British athlete has held the world record in javelin. That was Steve Backley who held the men's world record on three separate occasions, the first being in 1990.