Your best idea would be to get a professional electrician to look at it. The cable sizing depends on the installation method, load and length of run.
Basically you are installing something with around the same loading as a 10KW shower so your consumer unit and main fuse should be able to handle it ok but you may want to be careful on what else you are using at the same time as running the plasma at full current.
Yes the plugs and sockets do start to get expensive at that rating. We used to get a specific 125A 3 phase outlet unit for some of our welding machines and they were scary expensive.
Do you need such a large current for what you are cutting? I have a smaller Jasic 45 plasma. Nice little machine with a 5 year warranty. It doesn't have the same output or duty cycle as the hypertherm but I don't use it that much but it is a lot cheaper. Just a thought.