By James Davey and Paul Sandle LONDON (Reuters) – Britons splashed out on premium and party food at Tesco and Marks & Spencer over Christmas, prioritising treats at home over eating out, and favouring the biggest retailers for their festive spending in still tough economic conditions. Wider industry data had showed lacklustre sales over Christmas, fuelling concerns that the economy had tipped into a mild recession, but results from the country’s biggest names on Thursday defied the gloom. At Tesco, Britain’s No.1 supermarket chain, snacks and party food sales jumped 13.5% and sales of its top-…