The definitive OTP ( Off-The-Peg ) List (<£500 and <£1,000 sections)

I dont have the tools / space to build a bike here :(

...is it better for me to slowly accumulate all the parts and then build it...
On the basis of the above, "no".

With respect, if you don't know what you're doing then you're going the right way about blowing in excess of your budget and ending up with a bunch of incompatible parts, rather than a bike you can actually ride.

Unless you have a "parts bin" accumulated over a period of time, then building a bike from scratch is rarely the budget option.

Buy bike >>> ride bike >>> maintain bike >>> learn bike >>> build bike; in that order.