180A with a decent duty cycle (60%+) should be sufficient up to around 8mm thick. A 200A - 250A machine will have plenty of head room as you'll only use 100A - 150A for most DIY jobs. The extra ooomph may come in handy when you have some thick plate 10mm+ or your local farmer drops his tractor bucket on your front lawn for you to repair. I see the Oxfords are now £600. I paid £400 for mine 20 years ago and it paid for itself the first week due to my first car being a Vauxhall Nova which seemed to need new sills and rear wheel arches twice a year.

What sort of budget are you working to?