I would expect they’re doing it to try to disguise what the pizz poor hourly rate is by giving you what the take home pay is after certain tax allowances have been applied like meal allowances etc. They’re praying on the ignorance of people when it comes to income tax. Yes you’ll pay more if you do more days but no different than if you got paid more for the days you’re currently doing…when you’ve used up your personal allowance you’re paying 20% income tax on everything then up to £50,700 regardless of how many hours or days you’re working.
Generally companies that need to do this tend to be crap payers. Stobarts were the one in GB that immediately come to mind. When advertising what the hourly rate was they’d not put the hourly rate but the equivalent when applying tax free meal allowances etc so they could claim they paid £10 odd an hour when it was actually £9.50 plus £15 a day tax free meal allowance.
If a company refuses to state what their hourly rate is clearly without any shenanigins like “after tax” or “equivalent hourly rate” then I personally wouldn’t want to work for them because you’re already starting from a position where the employer is being sly, underhanded and feeding you crap before you’ve even started to work for them.