I thought originally you were asking about how free accommodation was taxed?
If you are paying for the accommodation provided it’s different – there’s an amount the government sets that the employer can use against your wages then if it’s any more than that then the difference comes off your wage.
So in your example the £280 a week would come down by £65 to £215, leaving you an hourly rate of £215/40. If that’s below minimum wage then it’s naughty. (I think it’s £35 a week they can allow for accommodation)
Probably accountant time though, or a have a good pour over HMRC online stuff – probably better I’m not sure I’m correct as never really had to deal with that scenario.