A groundsheet is, of course, essential with the hexamid. If you use it inside the netting, it doesn't have to be a proper bathtub as the netting will keep it up at the sides. A piece of cheap polycryo will do. Personally, I alsouse a piece under the netting as well, but that's partly because weoften camp on dirty ground. In total, our groundsheets come to about 125g.
I don't think rain coming under is too much of a problem as a solo tent. With 2 people, however, there is less room to get out of the way. For our use, where rain is only an occasional risk, it's quite easy to lower the tent to keep out the splashing. (Or so I think. We haven't had it in rain yet, but I have tried different pitches under a hose pipe.) However, in the UK, where you might get days of rain on end, that might not be ideal.
I've also read comments elsewhere from a lot of people in the UK who don't like net inners because of the draughts. The Hexamid would not be for them. I think it's very good if you want something that gives you more protection from rain, wind and insects than a standard tarp and if you are prepared for a bit more of an open experience than with a two skin tent. Given that you don't need a bivvy with it, the weight works out very well even compared to a tarp.
One other thing, it's very quick and easy to erect.