1)I've used acrylic myself and found it pretty easy to get good bubble free joins... The company I've sourced is an aerospace industry, I'm sure they know what they're doing, people in the aerospace industry are very exacting in their demands and for very good reason.
I know of no solvent available to the public that gives bubble free joints (and if they do it's at the cost of bond strength), They've always been found wanting, if you know of one that doesn't I'd very much like to hear about it.
2)The acrylic is computer cut, no issues there.
computer cutting is not the issue, making sure it's level FLAT contact on bonding is, it's a very difficult thing to do. That's why only specialist places can do it, it's not something that someone with a little workshop can do, unless we are talking about small stuff like your 12" cubes
3)I spoke to them about this before I found out it was cheaper to let them build the cubes, they're not allowed to use any glues that aren't tested by the government to ensure strength and chemical properties under load and their stress thresholds, this information then has to be published so that strength and stress ratios are available to the end user and to make sure the correct spec glue is used.
is this glue available to the public. Releasing bond strength figures is one thing, releasing chemical composition of the solvent is another.
And I really don't know what a 12" opti white cube should be £160, can't you get the glass cut and do it yourself. It would be a whole lot safer and easier than working with acrylic.
Ebay has 25cm cubes as well:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=250567293442
For 12mm we are talking about £15 a foot square which in the case of your tank would be £75 in materials already!
I see that they are 6mm thick acrylic and assume they're not braced.
acrylic bending is a whole lot worse than that. Unlike glass, when acrylic bends it "keeps" the bend over time, which means if the tank you see is already bent for some time, if you empty it completely it still retains the bend unless acted on to move back to the flat position.
I had a 24" acrylic cube tank what was braced in 10mm acrylic for £200 some while back.