If you leave your job without notice can they withhold any wages owed to you ?
Yup, but they can only withhold your notice pay. If you have 1 month notice in your contract, they can hold that. They can’t withhold any accrued holiday pay unless you owe them money or have taken more holidays than accrued.
H.
Yes they can, but they cannot stop you going “sick”, getting your pay, then putting your notice in & not going back. Depends on if there’s a chance you may want to return in the future.