Hi and welcome to the forum!
If your looking at the components then they are for building yourself, but OcUK do have quite a wide range of PC where by you choose the spec and they put it together for you.
If you build yourself then you get a greater selection of parts and get more for your money (or save more) if you use a system builder then you have the reassurance of it all working, being comptible and an overall warranty.
If you build yourself then the components will have their own warranty per part, all with at least a year, some can be lifetime and other for several years! Although anything over the first 12 months will be with the manufactuer of the part not OcUK.
Another thing to remember is that if you build yourself and damage a component, thats your loss. However nothing compares to building your own PC!
If you go down the build yourself route just ask here and someone will be sure to let you know if its all compatible and a good spend of your money before you buy. And any issues with putting it together again you can always ask for advice. Just do some research into building & compatibility... gotta love google