For what you want to do that PSU will
probably be OK - it does have two PCIE power cables after all. But compared to 750W PSUs from reputable PSU manufacturers this "OCUK" unit does look underspecified and low priced (which tends to indicate lower quality).
For any of the cards I mentioned above, it will probably be fine.
That said, if you haven't already bought it then don't buy that OCUK one - there are much better quality PSUs for similar money. The ones Uncle Petey mentioned are great - I would also suggest
this one.
Is there any advantage to running two weaker cards in sli/xfire or is a single card going to be a better bet?
As for running two card in SLI/CF instead of a single card - the general consensus is that you are best to get a better single card than two slower cards. Since in the 2 card case you are at the mercy of SLI/CF scaling, driver updates and possibly microstutter. Plus running two lower-end cards tends to use more power, create more heat/noise and restrict upgradability compared to running a single higher-end card.
Also, at this budget (£50 per card maximum) you won't be able to afford much of a gaming card if buying new (
this seems to be the best) - so second hand would be your only practical option.
So overall I would recommend sticking with a single card.
The 7770 and GTX 650 also appear to be running at higher clocks than the 7850, is the 7850s advantage purely because of processing cores?
Yes, the HD 7850 is running at a lower core clockspeed than the HD 7770 (860Mhz vs 1050MHz), however the HD 7850 has many more steam processors than the HD 7700 (1024 vs 640) - hence the 7850 performs much better in all tests.
The Nvidia GTX 650 uses a completely different architecture than the AMD HD 7000 series so you can't do an apples-to-apple comparison just looking at things like stream processor count and core clockspeed. Instead you need to look at real-world tests (such as the anandtech link in my first post which compares many cards in many gaming tests) to compare the performance of Nvidia cards with AMD cards.
Other question, would it be a good idea to fit a better cooler to the one that I go with?
It really depends on which card you want to go for - if it is a HD 7770, GTX 650 or one of the other cards that comes with a nice cooler anyway then I would be tempted to stick with the cooler that comes with it and only consider changing it after living with it for a while. However, if you go for a relatively hot/old card (like a GTX 470) then an aftermarket cooler is a very good idea.
