TV IS ABOUT TO BECOME HOME TO A pair of unusual British detectives. First, the medieval. On PBS, Sir Derek Jacobi (I, Claudius) portrays Brother Cadfael, the 12th-century Benedictine monk and hero of a popular series of mysteries by Ellis Peters (the nom de plume of Edith Pargeter). This new Mystery! entry, which airs over the next four Thursdays (9 p.m. ET), beginning Jan. 12, is a sort of Murder, He Scribed, illuminated by Jacobi's lambent talent. Now, on to the modern: Three times over the next few months (beginning Tues., Jan. 17, 9 p.m. ET), Robbie Coltrane returns to A&E as Eddie "Fitz" Fitzgerald, the self-destructive but shrewd Mancunian criminal psychologist. His dicey adventures in the Cracker series of films make for drama as gripping as any you will see on American TV this year. In his first outing, Fitz is after a soccer fan who has come murderously unhinged.