Little update on my situation:
GameStop.ie had ZSS and Dark Pit up for pre-order yesterday. I ordered both (2 ZSS's actually, one for a friend).
Amazon UK literally JUST had all of the EU June Wave up, and I managed to pre-order Ganondorf and Palutena there. The pre-orders for all of them went in minutes, though.
The only ones I haven't got pre-ordered (not including ShopTo, where I have all of the May and June Smash Amiibos still pre-ordered - they're merely my expensive back-up option) are Greninja and Jigglypuff. I'm hoping to pre-order them in-store at GameStop, as well as Palutena and Ganondorf (they're not that expensive on Amazon UK, but I'd still like to cut the price down as much as possible).
So yeah the upcoming six are going much better for me so far compared to the April Wave.