My daughter renewed hers around 3 months ago. On line application with old passport sent by signed for delivery. Just over 2 weeks from application to getting both new uk and old eu passports back.
Old eu passport had 5 months left to run. New uk passport valid for 10 years from date of issue - the remaining 5 months of her old passport were not credited to her new passport. With the 6 months validity rule most people end up with a 9 1/2 year passport for regular travel habits.