The key benefits of the 580EXII over the 430EXII are power, and wireless E-TTL II (auto exposure) master control over off-camera slave units (the 430EXII has slave functionality but not master functionality). If you don't intend to expand into shooting with multiple flashes, and if the Jessops unit was powerful enough for your needs, then the 430EXII will suit you fine.
If you're considering a 430EXII, dedicated third party units also worth considering are the Nissin Di622 MkII, Yongnuo 565EX or 468 II or the Metz 44AF1, 50AF1 or 58AF2. All of these more or less replicate the key features of the 430EX II, some with higher max power, some with less.
If you're considering a 580EXII, third party units worth also considering are the Nissin Di866 MkII or Sigma EF610 DG Super. Both are roughly equivalent to the 580EX in functionality and power.
Some of these third party units have USB-updateable firmware, some don't. Those that don't are more at risk of obsolescence with future Canon camera bodies. Canon's flashes don't have this issue.
