I think you can go up to about a 160 amp welder on a 13 amp plug, although the manufacturers tend to suggest one of the dedicated round blue sockets. I'd definately put a dedicated line in for a 200 amp welder. I'll try to find some regs.
I have ran my 200 from a 13 amp spur on a cooker circuit. Okay on all settings except flat out.
It will go flat out, but will blow a few fuses in the plug on the starts.
Ideally if you are going to use it a lot, you should have a seperate circuit with the 16amp round plug as Malc said.
Don't forget that if you do it yourself you 'should' get a registed spark or building control to sign it off.
He said the best setup would be a 30amp breaker in your main consumer unit...feeding another 3way cu with 6mm2 cable.
Then a 5amp breaker for lights, a 20amp breaker feeding a 4mm2 radial circuit for normal sockets. And another 20amp breaker and round pin socket dedicated to the welder.
Apparently anything above a 3kw load has to have its own circuit.