Sorry, I’ve not subscribed to any WWF plan, so can’t speak from experience. However, looking at the plans, the difference is an increase from around £8 per bottle to £10 per bottle on average.
Looking at a recent Decanter info-graphic about what you are paying for in a bottle of wine, you are getting almost twice as much value if wine when you go from £7.50 to £10.
Not only this, but the jump in bottle price will significantly increase the choice available (currently 166 wines under £8 on TWS website and another 200 odd between £8 and £10) to whoever puts these selections together so you will not only end up with a better quality bottle, but also a more varied selection.
If you’ve got the funds, looks like a good decision (speaking from ignorance, of course).