Mortgage activity climbed to its highest level this year in May, up 25 percent on January, according to the most recent National Mortgage Index from the UK’s leading independent mortgage broker, Mortgage Advice Bureau (MAB).

Using data from more than 500 brokers and 800 estate agents, the National Mortgage Index found the total number of mortgage applications year-to-date was up 16% on 2011. This increase is being driven in part by a surge in remortgage business, with the amount of remortgag...