How much tax can you save if you invest £5100 in a UK ISA?
The short answer is that you'll save precisely the tax that you'd otherwise have paid on the income from the deposit - i.e. 20%, 40% or 45% depending on your tax band.
If you shop around, instant access ISA rates are somewhere between 2%-3% at the moment, so the total interest will be around £100 - £150 making a tax saving of somewhere between £10 and £75 per year depending on your overall income.
However you typically get worse rates of interest within an ISA, which complicates the situation further. You should compare is the rate after tax that you'd get elsewhere with the rate before tax in the ISA, to get a fair assessment of the "savings".