As you know, Undos are not permitted at the Scout Hut on Saturdays,  but if you use the two taps facility in Settings you will not need an undo.  
Whether bidding or playing a card, tap once on either the bid or the card. If you have selected the wrong bid, or the wrong card, tap anywhere on the table, in the case of a bid, the bidding box will reappear, if it's a card, the card will return to the hand.  You can then make the correct bid, or play the correct card, again by tapping once, then confirming with a second tap.