If building is truly cheaper than buying on your side of the pond then it will help you to visit the build calculator on garf.org it'll give you a basic cut list and thickness requirements so you can call glass shops for quotes.
In the uk best bet is aquarist-classifieds.co.uk you will find loads of second hand tanks on there. no way is building a tank cheaper than buying. i have seen really good tanks on there for less than the cost of the silicone!