Hi Lee 
Habito would be my pick 
I’ve not used their service yet but I really like their ethical approach to providing their service, plus the fact that according to their research (alongside the University of Manchester), 55% of people were overpaying on their mortgage, by an average of £294 per month which seems like a big opportunity to me! I’d recommend listening to this Rebank podcast interview with their CEO, if you’re interested in hearing more about them.