I take it you are refering to standard springs with standard shocks? tansard springs and shocks were fitted to your car with the job of soaking up all the nasty imperfections the road throws at you. They are of course built to a budget, which is crap really and if you start messing about with crap, you end up with even crapper.

I bought my focus with supposedly sports suspension only to find it only had lowered springs. All the rest is standard. When I came up to a dropped drain cover(only about 1 1/2 inches), it felt like I had no tire on the rims and sounded like the srut was going to come through the top. When I took it down to my local special/race car builder, he told me it was because the struts could not do their job properly because they ran out of stoke and was hitting the bump stops as the struts effectivly have had about a qtr of the damping taken away. What I am now going to do through their advice(when funds allow)is to get a complete suspension kit that is matched to my car, that wont bottom out because the shocks will be matched to the springs.

I dont know that great deal about car suspension, but only my experience with just lowered srings, which I can live with ATM because I just have to stay away(most of the time)bloody big pot holes and drains that have sunk. I know you want to save money, but I really think you would be taking a big risk if you chopped standard springs and you might find your insurance would be affected. Also Im pretty sure the car would fail its mot. Dont let it end in tears.