I think the follow on milk at 6 months is a bit of a gimmick as they aren't allowed to advertise the first stage due to the government backing BFing only! Which is so stupid imo, but anyways, when you check the ingredients they're not really that different. So I guess that the companies have got round the anti-first stage by promoting a follow-on milk at 6 months to get themselves noticed.
I've read and heard HVs say that there is no need to swap from stage 1 to follow on. I guess the main advantage is that you can get tesco and boots points for follow on and also use points to buy them and get coupons, whereas stage 1 is treated as though it doesn't exist