I don't think Jet2 will bat an eye either. I don't fill my bike bag with all the rest of my luggage, but given that I can't carry it more than about 100 yards I'm sure it's well over 20kg.
I've never had a problem with them. I pay my £50, lug it over to check-in, they weigh it to get the overall load weight for the plane, then they tell me to bugger off to special Xray along with my rucksack. End of, as I believe people say these days.