Brits celebrated an 18th birthday last week — the 18th garden party held by the Los Angeles branch of the British Academy of Film and TV Arts.Some 300 members, including BAFTA/LA chairman Gary Dartnall and thesps Tippi Hedren and Olivia Newton-John, helped the sold-out event come of age. It was again held in the garden of the Hancock Park home of British Consul General Peter Hunt where guests sipped champagne, noshed on nibbles and took part in a raffle and silent auction. The bash is a victim of its own success. Last year’s event was so full, BAFTA/LA had to restrict its size this year. “We were turning folks away,” said Dartnall.