the problem with buying these pre built systems is that they over charge for whats in it.
other problem is once opened and booted and setup GOOD LUCK trying to send that back to the company. they dont touch opened goods.
GPU wise i would go for GTX970 or above.
Power supply wise go for 550w+ Gold.
http://www.scan.co.uk/products/650w-evga-supernova-650-g1-full-modular-80plus-gold-135mm-fan-atx-power-supply
something like that will do.
Also bit of advise. dont go for the "Reference" cards. go a bit higher for pre overclocked. look at clock speeds of each GPU and go for one with nice speeds.
eg: reference card clock 1042mhz, the some others will have 1104mhz so on and so forth.
Lastly for next build.
use www.scan.co.uk
www.ebuyer.com
and other sites like this to build from scratch.
for that cash and the cash you now want to put into it to make it gaming ready which is about 300 to 330 quid. You could have scraped together a new higher end CPU/mobo with DDR4 ram solid state and few other tweaks.
Happy hunting