I am in SW too is extremely difficult- I have just secured a post for sept in yr 1/2! This is a nice idea for capacity lesson - i know someone who did this for an ofsted lesson (for yr 1/2) and went very well. Just be sure to think about your differentiation. Check with them how many children whether mixed ability and whether you have a TA to plan for. Get some AfL and ICT in where you can too. Good Luck!