I just did a dummy run for your dates based on 2 adults/2 children 20 kgs luggage and pre booked seats (so no meals as not required and 20 kgs luggage but you could opt for more) and the difference is a rather sunbstantial £495 without taking into account any cashback which is a very good saving imho given that you're unlikely to use the airmiles.
So just one more observation is seatplan layout of the plane lots of 3's and a couple 2's with plenty of choice at the moment but would the Virgin airbus layout work better for you 2/4/2 in economy? See HERE
and maybe ONLCI is a bit of a bonus?
However, since doing the dummy run and seeing the actual monetary saving and how it suits the fact that you won't use the miles or added extras that VA provide then it seems to be a swaying factor towards choosing TC in your case.