British nationals must have a valid passport to travel to Saint Lucia. Their passport should have at least six months’ remaining validity from the date of entry. Travelers also need to present an onward or return ticket.
A visa is not required for British nationals. They can stay up to 42 days without a visa. The authorities of Saint Lucia take overstaying as a serious offense. Those who wish to stay longer must get permission from the local immigration authorities.
Additional information on entry requirements you may contact the St Lucia High Commission in UK.
St. Lucia High Commission