FOR THOSE WHO PREFER THEIR TV with a British accent, here's a redoubtable twosome. "Blue Boy," the season opener for PBS's Masterpiece Theatre (Sun., Oct. 2,9 p.m. ET) is drawn from an old Scottish ghost story. The ever-evocative Emma Thompson stars as a pregnant woman staying with her unfaithful husband (Adrian Dunbar) at a resort haunted by a boy who drowned in the adjoining loch a century before. Then there's Anna Lee: Headcase (A&E Tues., Oct. 4,9 p.m. ET), the first in a bracing film series based on Liza Cody's novels. Imogene Stubbs stars as the title character, a thoroughly modern girl who has left the London police for a private detective firm. In the debut she's helping a disturbed teenager who is suspected of murder.