There will be no alternative ways than a diamond pass. The Diamond Passes are usually called by number - with the current selling speed you'll probably end up on Sunday.
Normally tickets are non-transferable.
Thank you that's helpful. My best case scenario is that I've either had the baby or am still waiting. So if that's the case could I just buy entry for the Sunday? Or would I have to buy Saturday as well on the off chance my number is called early? Or by then will you allocate x amount of signings to each day? I can't imagine travelling down to London so heavily pregnant for 2 days, even if I can make it!