I would go with off the shelf Konis with ground control coilovers. Only lower the car about 1.5" and get custom rates with your ground controls (which shouldn't cost any extra) For the street, I would go with 225-250# - 5" springs up front and 300# - 6" in the rear. This setup should cost no more than $425.
The Konis should run no more than $550 for all four corners, plus shipping to the UK.. Thus a total with shipping at or around $1,000, which with the exchange rate, should be equivalent to about $950.
The Koni fronts are inserts and the rears are full replacement struts. They're both easy to install, with the fronts being slightly more involved.
My reasoning for this is that the Konis are guaranteed for life as long as you don't run them lower than 1.5" below stock, and if they start leak at all, they'll send you a new one, and have you ship back the bad one back.
Also the ground controls allows you to adjust the ride height, which may be necessary with 18's. The other nice thing is both the Koni and Ground Control setups are very, very easy to sell on these boards if you wanted to go back to stock.
I hope this helps. - Aaron