Last year, we started down this road, wanting to replace an old Bosch 12V drill and driver set. We found 3rd party batteries to keep us going, and did a lot of research, noting things like cross compatibility and suchlike. We came to the following conclusions:
1. Most of the 'own-brand' makes are made by the same Chinese company (Positec), and have the same battery configuration. They probably work just fine, with the advantage that batteries are widely available from a number of badges. Positec's own brand is WORX, and they do a system where they all take the same batteries.
2. The only company which seem to have always used the same battery connection system is Ryobi.
3. 18V seems to be a sweet spot for how much bang you get for your buck.
4. Price is everything. If Toolstation or Screwfix are having a deal, that's probably your best bet.
We ended up with a DeWalt drill/driver + impact driver combo because we got it on a Toolstation deal where it was £100 cheaper than anything else that was equivalent. True, we're locked into DeWalt for the foreseeable future, but that's not the worst thing that could have happened. I still have my 'made in Switzerland' Elu router, which is the same company, and you can still buy parts for it.
I'm pretty sure that had the price been right, the WORX stuff would have been perfectly adequate for what we want.