"Is it possible to build a DECENT Laptop for under £300?"
It's almost impossible to build one at all. There are specialist sites that supply laptop components, but for all practical purposes, forget it!
If you want to build a computer, stay with a desktop. There are plenty of sites that offer in depth tutorials and no doubt, there's plenty of advice available here.
If you're including a screen in your proposed budget, then it's tight, but possible.