For vibrations, yes, a Subdude can help. However, for what they cost, they're surprisingly easy to make yourself for less than sticker price. Really, all you need is a slab of MDF as a base (though plywood could work in a pinch) and dampening material underneath. Firm open-cell foam (meaning difficult to compress, not rigid and inflexible) is what's in the Subdude, but "acoustical foam" isn't going to offer any special benefit in a DIY situation, so you can rifle through whatever you have in the garage or closet. What you're after is stability without excessive compression of the foam. If you don't happen to have these materials on hand, you should still be able to purchase them for less than a finished Subdude.
I'm not too familiar with what's offered in the UK, but I can't imagine it's drastically different than the US for such a basic project. The aforementioned Sorbothane can be had for a reasonable price here at least, as can foam. Actually, if you know how much each speaker weighs and their dimensions, I can figure out the dimensions and foam type you would want if you decide to go the new foam route. Of course, for convenience, you can't beat a pre-made product, and the Subdude does do what it claims. But if you don't mind spending a little extra time gathering materials you can save a few bucks.