Ah jeez - you're being ridiculously oversold.
First off (and with apologies to the enthusiasts here), go over to
http://www.mig-welding.co.uk/forum/index.php and ask the question of people who understand what you're looking for and what will do.
Clarke MIGs, for what you're looking at, will do it until the cows come home. Don't worry too much about duty cycle because the absolute worst thing that happens if you abuse it is that the thermal cut-out kicks in and you go have a coffee for 10 minutes.
For 3mm steel a 130A or 150A is ample. Any Clarke 130 from
MachineMart will do that. Check ebay for something maybe £75-£100 cheaper.
Don't go with gasless - always go with gas and either start with the hobby bottles or go straight to rented bottles. Beware that these will still cost you £50 deposit plus £25 for the gas.
Sealey are also excellent units but, to be honest, for the money you're talking, look at Clarkes.
Eurotorches are nice-to-have but, for a hobby welder, what ships will be fine.
And you can plug a 130 into a standard 13A socket.