At this price point its not necessarily the sort of synth I would expect to see in stock at many retailers. I think they will often order from their distributor when a customer orders from them.
But I may be wrong about that, eventually, its sort of hard to tell because supplies have been pretty limited overall since production started to happen properly last summer. A lot of people pre-ordered up to a year earlier, and it took time for them to receive their Quantums.
Having said that, what I did was order one from a UK supplier at the start of last August, and I received mine by the end of August! This was not typical at the time and I got very lucky, especially as supplies were even more limited back then. But this is still what I would recommend now, especially if you choose a supplier that wont actually take the money from you until your Quantum is about to ship. I used gear4music.
The only ones I've seen in stock have been 'b stock' ones where someone returned one or it had been out on display or whatever. I actually tried this option last summer too, when there was one in stock at a German company that shipped to the UK, and this worked too, causing me to accidentally end up with two Quantums! (I then sold one of them)