I highly recommend a trip to WIM to get your suspension geometry set correctly, the lowering will completely destroy the settings. If you haven't been to WIM before then your original settings won't have been good anyway, the OEM geometry causes premature tyre wear and non-optimal handling. This is a lot more than just setting the wheel alignment, so specialised equipment as well as chassis expertise is needed.
Have a look here for more details on WIM.
Have you got a rough idea as to how much this stuff is likely to cost in addition to the £94 for the springs? Spring fitting, WIM working their magic etc