As
@stephen1546 said, Pinot Grigio is "crisp and fresh", which in practice means rather watery in terms of colour and flavour, and on the light end of the ABV range too. Pinot Grigio is usually bottled in clear glass and looks pretty much like water:
Did you measure the specific gravity of the kit before and after fermentation? That would allow you to work out the ABV to see if it really is watery in alcohol terms.
I've made quite a few white wines from Beaverdale, Kenridge Classic, Vineco Atmosphere (previously Kenridge Showcase), and Selection Eclipse, and in generally the more you pay the more juice you get (10L for a Beaverdale or Kenridge Classic kit, 16L for a Vineco Atmosphere kit, 18L for Selection Eclipse) and the more "provenance" the juice. In other words a £40 kit might include 10L of "Sauvignon Blanc" grape juice but a £120 kit will include 18L of "New Zealand Marborough Sauvignon Blanc" juice. It might not always mean a nicer wine, but if you like the particular flavour that a certain region gives then it's an important factor.
My advice would be to go up one step to the Vineco Atmosphere kits, which takes the juice up to 16L rather that 10L. They cost about £25 more than the Kenridge Classic kits, but it still works out at under £2.50 a bottle for something which is generally significantly nicer than a supermarket wine two or three times the price. That said, supermarket Pinot Grigio is often pretty cheap anyway, so a kit works out as much less of a bargain than, say, a Gewürztraminer (rarely under £8 a bottle in shops) or Amarone (£15 or more).