I gave 3 weeks notice to FR and I was glad I did. They chased me for this £5k for a few weeks, even asked me to write an email to say I would pay it, Because they know they can’t enforce it. I never paid. They can’t prove loss of earnings on you. The also can’t charge you a penalty for leaving either. It’s funny how their contracts are there to restrict you working as a “contractor”. If they treated your right you wouldn’t leave in the first place.
A few weeks into the new company, the HR manager called me to ask how I got round it as they were offering to pay this penalty fee to FR and they wanted a way of getting out of it.