What will you be using the rig for? By the looks of things I'm assuming it's geared towards gaming, so most of my advice will be based upon that, though it would apply to most other uses:
Personally, I don't think the extra warranty of the Caviar Blacks are really worth the added costs, especially since the performance is very similar to the Blue. Another option worth considering would be the
7200.14 Barracuda though, which is currently notably cheaper than the Blue and also faster.
Since you did ask about possible ways to save some money, you could get an 8GB kit for around £10 less, which won't affect performance at all. Units like
this or
this would be fine, however if you prefer the looks of the Vengeance, there's nothing wrong with getting that either.
The original HX series are decent, but they usually aren't worth the amount they cost. While I don't know how much the HX750 costs in the UK, I assume it costs around ~£90, in which case an
XFX ProSeries 850w or an
EVGA Supernova 750w would be better choices.
I wouldn't recommend the G45 if you plan on overclocking though, it's not really ideal with the 6 phase design, something like the GD55 would be a better choice overall, at least if you plan on heavy overclocking.
Lastly, for general use getting a high-end SSD really isn't worth it over a cheaper, but slightly slower unit given the real-world performance difference (boot times and application load time) isn't that large. The 840 Pro costs around £105, however you could save £30 by going for a
Sandisk Extreme or a
Samsung 840 instead.