for £700 you have a wide choice in the 200amps bracket. i believe portamig do a 235 amp unit for less than that with a bottom setting of 20amps (ideal for cars). I assume you also need a welding helmet as well with in the budget of £700. Send a pm to weldequip on the forum here as he is the supplier of portamig units which many forum members own including myself.
£700 is a nice budget! You'll get a cracking machine all-in for that, definitely something better than a Sealey. (not that there's anything wrong with them, just that there's better ones about).
Look for something with a decent wire-feed system, euro torch (pretty much everything in that price range will have that), a good range of voltage selections (the more the merrier) and a decent warranty .
I'm also a member of the Portamig fan club .
Well, I have been offered and bought a Cebora 180 amp single phase MIG and I have also bought a Sealey 150 Supermig. Both second hand and usable. Happy welding guys.