Will there be any surprises after I sign up?

If you sign up on 15 June, your included GB and monthly fee for the first month will only be half of what the selected plan says, because June has 30 days. If you signed up on the 29th, you would only get 1/30th of the included data and only pay 1/30th of the base fee. Once you are billed on 2 July, you would then have the full data included and pay the full base fee for July.