I'm from NZ so my go to is "mate" but having lived in the UK and Ireland for nearly ten years, holy crap do they have you covered with many fantastic options...
Darling - this is what old ladies at my local supermarket checkout prefer in East London. Dear - similar to above it is probably supposed to be said to someone younger than you Love - pronounced "Luv" is all England I think Boss - (or Bossman - but not relevant here) is a widespread London one. Duck - Often used in the Midlands, particularly around Derbyshire and Nottinghamshire. Pet - the North East, such as in Newcastle. My lover - the South West, particularly in Bristol and surrounding areas. Gaffer - maybe Irish?
The more obviously gendered ones are actually extensive too like Hen, Babs, Babes, Son, Chick, Flower, Laddie, Lassie, Bonnie, Doll.
Honourable mention for Twat - when you don't want to be polite.
People who grew up here will have even more. To them I say, have I passed the Home Office "Life in the UK" test now?
TIL he is Julian Casablanca's father (lead singer of the Strokes).