Perhaps you would consider buying some sheets of 18mm or 25mm ply, cutting some 800mm x 800mm squares, double them up, glue / screw together, paint with some matt black paint, pop in some t nuts, modify the and bolt the baseplates to that then add ballast / outriggers. The DIY jobby did well enough for us.
Building on what Peter suggested really. You may also consider sticking some rubber matting underneath it all to protect the flooring.
I've also seen barbecue heat plates modified in the workshop to accept the various conical connectors and pins a la Trilite / Milos. You're talking 5mm by about 800mm across. I would imagine this is fairly straightforward stuff for most metal workers, but the base plate itself would weigh 25 - 30kg and wouldn't come cheap.
In fact, a quick search brought up this which, in my opinion, is exactly the sort of thing you should be looking at. I don't know of any UK suppliers, so good luck with that.
Stage weights from Flints, or a few sandbags from B&Q!