By James Davey LONDON (Reuters) – As Britons face the worst hit to their disposable income in 30 years, Tesco is outstripping rival retailers by tempting more of them through its checkouts with a money-saving loyalty scheme. Tesco’s Clubcards are held by more than 20 million British households, 8.5 million of them via an app, boosting sales and helping Britain’s biggest retailer strike better deals with suppliers in one of the world’s most cut-throat grocery markets. Tesco, a pioneer in customer data science and analytics, launched Clubcard in 1995 to track shopping trends and reward loyal cus…