Hi...since you're in the UK, i'd check with Astec models for a nice hull. If i remember well they also sell Feigao brushless motors. I've bought a micro-cat hull from them recently, and although it's fragile, and requires some good planning before you actually start gluing stuff together, it's super super light. i kitted it out with a walkera 380m brushless and a graupner 40A genius controller. radio gear was also from graupner and then i installed an offset rudder i bought on ebay. All this stuff is available from ebay...BUT i still have no idea what speed this thing does. The fact that it's so small might make it seem like it's going faster than it actually is, but with 12 cells only the prop and rudder stay in the water.

So a rough guesstimate of the costs:

Micro-cat hull 17GBP
rudder combo 13GBP
Walkera 380M 25GBP (postage included)
graupner 38mm prop 3 GBP
Graupner RC C4 sonic - 38 GBP
Shaft, stuffing tube and coupling (graupner) - 7GBP
Graupner Genius 40A Brushless ESC - 70GBP

So all in all i got a pretty decent kit for a total of 173GBP....but you still need to add cells to that.....but that dependes on the run-times you want and how much you want to spend. with this kit you can go from 7.2 to 14.4v....but a lot also depends on what kind of cells you buy - lipo would be the way to go for minimum weight and maximum power....

A
Graupner Genius 40A -68GBP