There are a couple of points to be made here I think - some I already touched on in another thread where ox raised this question:
In terms of cost of consumbales / goods generally the UK is far more expensive than the US - we tend to pay in £ what our friends over the pond pay in $ - which makes the stuff in this country almost twice the price generally - its just a fact of life. But when we come to the US - woo hoo everything is just soooooo cheap 'cos of the rate of exchange !
One thing that the larger manufacturer's offer is designed units that generally look pretty neat in a house. Now I'm not saying that custom tanks dont look good, but the appeal of some of the all in one packages is their conveniance and their looks. Some custom tanks look amazing - but I'll bet you they aint cheap. If what you want is a cheap glass box with the best filtration etc. then custom is probably best for you. If you want something more designed for your living room, then you're probably going to look at an all in one job.
I have a juwel trigon - I wanted a corner tank which would look ok in our living room, I knew that I wanted to keep small tropicals, so whilst the lighting and filter may not be the best, they do all that I need to create a good stable environment for my fish and looks good too. I also bought it second hand so I paid £150 not the £400 plus it would be new - It was mint condition and only 11 months old