After umming and ahhing quite a bit, I decided the only really permanent benchtop material is Jarrah. This stuff is like the stainless steel of timber. Dense, close grained, very hard on tools, and really incredibly durable. Unlike MDF. I bought five 125mm wide, 30mm thick, 2400mm long planks and laminated them together, then trimmed the ends to give me a 2200 x 625 x 30 benchtop surface. I spent the best part of a day planing the top as flat as I could get it (whoa, upper body work!), then finished it with lots of carnauba wax.
Here it is, set up with my metalworking vise.