I've posted this link loads of times, but it's so damn helpful, I'll post it again.
This is an offset wheel calculator with a graphical representation of difference. You put the stock measurements in the top, then your new wheels measurements in the next one
http://www.morfed.net/wheel-offset-calculator/
You'll be able to see what the difference will be, as Jamie says, stock offsets are pretty tame, and makes the wheel appear to be set right back into the arch. Use the above calculator to see the difference.
Also bear in mind that width has an effect on offset. The stock rear is -50 with a width of 9.5, Obviously a 10 inch wide wheel with the same offset will not sit in the same place. Don't go buying 12 inch wide wheels with a similar offset and expect them to fit.
Also, just so you know, the wheels you're looking at will not accomodate UK spec brakes, I know you said your car is a JDM, but so is mine, and I have the larger brakes