What are the chances of getting day seats for Wicked the Musical in London?

The only way to be sure would probably be to get there for when the theatre opens. Otherwise, if you got there and the front row ones had sold, you could still get other tickets elsewhere in the theatre that hadn't already sold, and buying at the box office means you don't pay a booking fee, which would be a little saving. If you don't have luck that way but still want to get a good deal on tickets, the TKTS booth in leicester square (in the clock tower building) is run by the society of london theatre, so is okay, and a couple of websites I have used to get a good deal, including for Wicked are www.london-travel-centre.com, and www.discounttheatre.com, and www.lovetheatre.com. If you travel by train, the 2 for 1 london offers sometimes include musicals, so it might be worth looking into.