Who keeps the interest on tax paid PAYE?

I am self employed and pay my tax at the end of the year so I keep the interest it gets in the previous year.
However if I was employed my employer would take tax PAYE. So does the employer keep all the interest? Quite at lot for a large firm!

Yes, employers are obliged to pay deductions monthly, by 19th of each month. Months end on the 5th. So I suppose employer could get a fortnight's interest each month.
Needless to say the Government gets the larger slice throughout the year!

No, it is submitted to the government monthly. So they are the ones who get the interest.

Interesting that you can pay at the end of the year with no penalties. Here (Canada), if you do not have tax deducted at source (equivalent to PAYE), you have to pay quarterly installments, or face interest penalties.

Employers get a couple of weeks' worth of interest on the deductions but it's unlikely this compensates them sufficiently for the cost of being the government's tax collectors. PAYE is very cumbersome and entails a significant cost for business when it could be argued that it's really nothing to do with them.

Compared to PAYE taxpayers you as a self employed taxpayer do get to withhold your tax longer, and so can earn interest on it. But you also have to work it out yourself, rather than simply have a code whereby it's deducted without the employee having to do anything. Swings and roundabouts.