* Liberum analyst says sale price supports valuations (Adds details on sale, retail sector, debt): April 18 (Reuters) - Britain’s Intu Properties plans to sell a 50 percent stake in an East Midlands shopping centre to a Kuwait-backed real estate investment firm for 186.3 million pounds ($242.53 million) in an attempt to cut its debt. Cale Street Investments, which is backed by Kuwait’s sovereign wealth fund, plans to buy half the interest in the Derby shopping centre, which is visited by 22 million shoppers each year, Intu said in a statement on Thursday.