I also like Calshot Activity centre.
It has a wide (3-4 boat) slipway, plenty of parking, clean showers.
The slipway faces north – ie away from the solent and towards Southampton Water – which means that even on windy days, you tend to have reasonably flat water to launch into. (ie no breakers)
Also, the significant benefit which I personally like is that once you launch you are pretty much straight onto the solent. ie there is no long river to traverse which can be tricky if trying to sail with moored boats, and even more tricky if (like me) you have no motor!!
That said, there are some big container ships going in and out of Southampton, so you have to be mindful of these and stay out of the deep channel. This is easily do-able, though you may need to cross it.