I know one version of the T&C says mimimum of £300, but there are two other places, including another version of the T&C that say "OVER £300".

I would strongly advise doing £301 to be sure, if you are just trying to hit the minimum to qualify.

I recently got caught out myself on the direct ACE site by thinking an order of £500 TC would not be charged a delivery fee, when I looked back at the terms it said over £500, and I was duly charged £5.