Hi
For standard ticket holders if your bag does not fit under the seat in front of you will be charged £40. You can only take a bag 45cmx36cmx20cm which must fit under the seat. There is no weight limit.
Only those that purchase a seat at the front of the plane or extra leg room, will be able to bring a larger bag to go in the overhead lockers. These tickets cost between £7.99 and £39.99 extra.
Geoff