You do not need to upgrade the PSU to play BF3 and Crysis 3 but I do note that for £700 the PSU although seems to have a decent amount of watts (650) it does not seem to be a named brand which I would expect for that sort of cash.
Also the ad says that the CPU has been over-clocked to 4.6Ghz yet there is no mention of what heatsink/cooler comes with the PC. A lot more heat is generated after an over-clock and I for one would want to know that there is a decent cooler in the rig.
For £700 it comes in a pretty cheap case (around £35)I would expect more.
I do not believe that this build is worth £700 but I build my own so perhaps am not best placed to judge whether this is good value or not.
As to the games you mentioned. For BF3 this is the sort of frame-rates you will get. BF3. Crysis 3 should be around the same dependent on monitor resolution.