Roymondo:
Whether they get 150 quid a week or 30k a year tax free is irrelevant - my point was that the tax allowance is based on amount earned, not the number of days worked (which was the assertion I commented on).
No, it’s still not.
If you earn £150 a week, you’ll pay no tax under PAYE.
If you earn £300 a week, and offset £150 of it, you’ll still pay no tax.
If you earn £300 a week, and offset none of it, you’ll pay about £50 tax and NI.
If you earn £9k a week and offset all of if, you’ll pay no tax.
The last point means “earnings have nothing to do with it”.
MP’s offset more in expenses than they get paid in actual wages. This means it actually costs the exchequer more than double the headline rate to employ them effectively… 
Higher base earnings just means more lies on the tax reclaim sheet.
What gets me though is why we mere mortals are only allowed to claim things like “mileage” and “works clothing” where MPs get to claim their mortgage & rents, foreign holidays, getting the decorating & gardening done, and even having the houses extended…
Something is not right with the system when not one single MP has gone to jail for utter and deliberate fraud. 
The indivudual’s tax allowance starts off the same for everyone, and then gets certain addons/takeaways based upon their other circumstances, such as being married, living abroad, and even how much tax they owe/are owed from last year. This year’s ‘earnings to come’ can’t have anything to do with it again, because it is not known yet what they are going to be at tax year end. LAST year’s earnings don’t have any influence, beyond if an under or overpayment has been made, in which case the code will be modified to claw back extra taxes, or an over-paid rebate for instance.
An interesting point of note would be if an MP was sacked about halfway through the year, and did no paid work after that. He’s already claimed offsets for the entire year as an ongoing thing (like selling his house at a loss to his brother, then paying £9k per week to rent it off him, that £9k then being claimed back from taxes!)
HMRC might well ask him to forfeit ALL claims made, since they are clearly valid for the first six months of the tax year at best, but he’s claimed 100% of the year in advance… More fraud.
Meanwhile, Bod, Guy, and Myself get grilled about not retaining our meal receipts when claiming for the flat rate meal allowances! (Que “Sharp Intake of Breath”!) 