Forget the phony math. The facts of the problem are. The bill was 25£, the change was 5£. You each took 1£ leaving 2£ tip. 25+5 = 30.

Or to put it another way you each paid 10£ and took 1£ back making 3£. The bill was 25£ and the tip 2£ which = 27£ + 3£ you kept for a total of 30£.