Well, the obvious candidates for a heavy duty pipe slice would have to be the firms which make industrial plumbing tools, namely Rems, Rigid and Rothenberger (who between them make powered pipe threaders, crimpers, etc). To that you can add Irwin-Record who still make the old Record roller pipe cutters, and maybe Faithfull do a knock-off of this as they do with so many old Record products (don't know, but Faithful copies are cheaper than the OEM stuff). Either way these are very expensive tools (circa £150 to £250) so maybe looking on eBay would bag you a cheap secondhand one, or you could always hire one for the day (HSS and Jewson both list them)
Edit: Just found that Silverline do a lightweight cutter capable of cutting copper pipe up to 64mm (part no. 868825). This won't be heavy enough to cut your pipe, but at least it should be able to mark it. £14.80 on Amazon today