Are per diems taxed?

My husband and I are Americans in the UK on a work permit, he works for a UK company as an employee, not an independent contractor.  The company has sent him about 100 miles away for a job that will take approximately 3 weeks to complete.  His boss is giving him a daily wage of 110 pounds per day, and a per diem of 60 pounds a day to cover his accomodation and meals.  The per diem amount, however, is being included in his taxable daily wage (170 pounds pre-tax), so in reality, it will only be about 40 pounds a day after tax (I am guessing). 
Is this legal?  In my experience in the USA (perhaps my first mistake here comparing), per diems are not taxed because they are not earned income, and could push the employee into a higher tax bracket as well.  If this is indeed how they do it here in the UK, is there a way to claim back his housing and meal expenses while on this job, since it will in theory be an out-of-pocket expense coming from his salary?  If it is re-claimable, could someone let me know if there is a form we need to fill out to send to Inland Revenue?  Thanks for any advice, I am concerned we are being taken advantage of here, as we asked for the per diem not to be taxed and his employer got very defensive...

Using UK tax terminology we are talking about a "round sum allowance" and such allowances are taxed under PAYE. However that does not stop the person claiming a deduction for the actual expenditure incurred .... at 40p per mile for the distance driven on company business, and the cost of hotel accommodation and subsistence.

Your husband should keep notes about all such expenditure, and in due course submit a claim to HMRC for an adjustment in code number, or if then after the end of the tax year, a refund from HMRC.

Thanks so much for your response, that's really good to know; one last follow up please, is it better to contact the HMRC in May to request a refund with supporting receipts
(we don't anticipate any more jobs away this tax year) , or to contact HMRC once the job is complete for an adjusted tax rate?  Would this just involve writing a letter and including a pay stub and receipts or is there an actual form we would need to download and include (apologies if I am still in IRS-think)? Thanks again for your expertise, now we can stub grumbling about the boss trying to rip us off, haha

If there are going to be no more instances of this sort of thing before next 5th April, I think a letter to HMRC is called for once this 3 weeks is over. But don't send the receipts, they would not welcome that, but instead say in the letter that the receipts are available should they want to see them.

What your husband will be asking for is an adjustment to his tax code number for the current year. Business mileage at 40p per mile, and actual expenditure on hotel and meals.

Your husband is probably on code 647L at the moment. Any increase in that code will lead to a reduction in tax deducted from his pay under PAYE.

Thanks so much for this John! Experts like you really make the whole expat experience easier to navigate