LONDON—Britain’s economy unexpectedly regained momentum in October, despite surging costs and mixed consumer signals, according to surveys on Friday that could tempt the Bank of England (BoE) to raise interest rates for the first time since the pandemic. The preliminary “flash” IHS Markit/CIPS Composite Purchasing Managers’ Index rose by the largest amount since May to hit 56.8, up from 54.9 in September. By contrast, a Reuters poll of economists had pointed to a further slowdown to 54.0. Sterling rose to the day’s high so far against the dollar after the data, which contrasted with earlier figures showing a record fifth straight monthly fall in retail sales in September, despite panic-buying of petrol late in the month. However, IHS Markit’s chief business economist, Chris Williamson, said the unexpected rebound in the PMI should not be viewed as a green light for the BoE to raise rates on Nov. 4. “The service …