By Laura McGuire Britain’s biggest supermarket Tesco has upgraded its profit guidance for the year off the back of a strong Christmas performance. In an update to markets, the board said it now expects to make £2.75bn in retail operating profit, up from a previous forecast of £2.6bn. It comes after the ‘Big Four’ grocer reported a 9.2 per cent lift in sales in the four weeks to Christmas. A slew of price cuts was thanked for a drive in activity and Tesco said it is now consistently the cheapest full-line grocer for over 14 months. Spending on its clubcards was also up 83 per cent year on year,…