If you're not overclocking the CPU, I'd recommend just grabbing a cooler master Hyper Evo, to be honest.
Costs £20. It would still allow for some overclocking, and run quieter than the stock heatsink Intel supply. It takes up a smaller foot print, since it only comes with one fan, you can install a second fan.
Erm, the Phanteks 12DX... it isn't as large as heatsinks come. I actually own one, and it is cooling my Haswell i7, the 4770k. Does a fantastic job. It only covers the CPU socket, it doesn't interfere with the tall heatsink on RAM modules et cetera. So, I recommend it, but there are definitely other options. I would disagree with the H80i more than I disagree with the H100i. Based on its cost to performance ratio. You can achieve the same thermals for half the cost of these AIO coolers. If you want the H80 because of some sort of preference, then that's fine. As long as you get everything you need, right?
But yeah, you don't want to spend lots on a heatsink if you're not going to overclock, regardless of its footprint. If you're not overclocking, you could just use the stock heatsink, since that happens to be quite small.
As for GPU longevity: People bought a GTX Titan, because it was "all they were ever going to need". Then the GTX 780 emerged, which runs on the same GK110 chip as the Titan. And with both cards at stock speeds, the 780 offered anything from 90-102% of the gaming performance of the Titan, for almost half the cost. And the 780 was released a few months after the Titan. Buying a GPU long-term isn't always the smartest choice.
To tell you the truth, there is very little difference between the high-tier cards. The 7950/7970/670/680/770 are all pretty much neck and neck. It is quite possible to buy a 7950 that beats a 7970. It is "luck of the draw". You can get a good card, you can get a bad card. They all trade blows, with some games favouring ATI, and some favouring Nvidia.
Hope that all makes sense. And I definitely recommend XFX PSUs, like the other guy mentioned. However, you don't need 750W for the motherboard you are using. You can only use one graphics card on that LX motherboard you've chosen. 750W is appropriate for two GPUs. It might be an idea to pick out a different motherboard. For an extra £20, you'll hit the £100 price point. You can find plenty of decent motherboards that would allow 2-way configs with crossfire or SLI support. That's just something to consider, that might prolong the life of the system.