Available not just on weekend days, but right to December 31st, All standard tickets to Up (2d version only playing) are available for 95p, plus a booking fee of 80p/ticket (rip off!) if booking online.
This is part of the Kids AM promotion, so you have to take at least one child per adult (I think)
However, this is likely to get very hot, so suggest you don't take the gamble. When they did this offer with Ice Age 3, all of the tickets were sold within a few hours of this getting out to the different moneysaving boards!
70-something retired balloon salesman Carl Fredricksen (voiced by Edward Asner) decides to use his twilight years to fulfil his lifelong dream of travelling through the wilds of South America. Tying thousands of balloons to his house, he lifts off for his big adventure - only to discover that he has inadvertently brought a stowaway along with him in the form of Russell (Jordan Nagai), an eight-year-old wilderness explorer who is desperate to earn his 'Helping the Elderly' merit badge.