Assuming your car isn't lowered and you currently have genuine Monza Shadows on your car (which are 7.5j x 18", ET51, with a 5 x 112 bolt pattern), wheels that are 8j x 18" with the same 5 x 112 bolt pattern and a 45 offset should fit OK; the outside edge of the wheel will be an extra 12mm closer to the wheel arch edge.
The ad says they are also available with a 35 offset. This might be a bit tight; they will protrude an extra 22mm compared to your Monza Shadows (don't know for sure, but they may extend beyond the wheelarch edge?). Maybe if other forum members have fitted 8x18" ET35 to their Golf, they'll advise you if they fit without fouling the bodywork.
You might find this offset calculator useful; input the width and offset of both your new and old wheels and it will calculate how much more or less clearance you'll have from fitting different sized wheels;
http://www.1010tires.com/Tools/Wheel-Offset-Calculator#Your tyres will fit; if you scroll down the ad, it states that the wheels can be supplied with 225 / 40 x 18" tyres, which are the same size as yours.
Bear in mind that a set of 4 brand new alloys for £399 (less than the price of one new genuine Monza Shadow) are unlikely to be as good a quality structurally as the original Monza Shadows, so could be more prone to buckling and cracking from impacts with potholes etc. Also, if you do fit different wheels to your car, this is classed as a modification for insurance purposes, so ensure you notify your insurer, otherwise you could invalidate your insurance.
