Anyone received theirs yet? I ordered mine at about 16:03 on 8th March and it is still on back-order, although I've now had two messages from Vodafone saying they are working to get my order to me and apologising for the delay.
From reading the comments at hotukdeals, the easiest way to cancel appears to be to go to the online chat once you receive the router and sim card (which arrive together) and say you want to give your 30 day notice on the SIM contract as you only need to use it for a month.
This may turn out as needing to pay for month 2, and then getting a full (or almost full, depending on when you give the notice) refund at the start of month 3.
You need to be careful this isn't misinterpreted as you wanting to take advantage of the 14-day cooling-off period to completely cancel the purchase (in which case they will request the router back and give you a full refund).