The main benefit of the extra speeds of Infinity2 (80/20) over Infinity1 (40/10) are for big downloads, and uploads for backup. If neither of these applies to you, Infinity1 may be quite adequate.
You may find you stream more HD video (YouTube, iPlayer, ....) so your usage may go up quite a bit even with Infinity1. It certainly may as the kids get older .... Unfortanately, impossible to give a more coherent answer.
I would definitely go for an unlimited package whether that be Unlimited Infinity 1 or Unlimited Infinity 2 is obviously your choice. The reason I would go for unlimited is that you will no doubt use more of the limits as time goes on, whether this is on your TV, smartphones or tablets and if you are unlimited then there will be no nasty surprises if you exceed the limit.
As there is only £3 of a difference between option 1 and option 2 I would go for option 2 which gives you a bit more speed which will be of benefit if more than one person is using the Internet for say watching a movie or playing games etc.