If something is £139.99 and I get a 10% discount, how much would the price be?
or even 125.99
No it isn't.
10% of £139.99 is £13.999 which has to be rounded off to two decimal places.*
So, the discount is exactly £14.00.
The original price less the discount gives £139.99 - £14.00 = £125.99
*Actually it is up to individual store policy whether any discount is to be rounded off, rounded up or rounded down.