There’s plenty of options, there also plenty of opinions about wether or not you have invested in the wrong system. Your best friend is the community, have a search, lots of info in here. ST gives you plenty of flexibility from lots of vendors.
Sockets.
Couple of pointers. I’m not aware of any UK replacement sockets. If anyone wants to correct me I’d love to hear it. If you want a ‘pocket’ like the ST socket then you can use the stuff from TKB Home available from vesternet.com (good site), they’re z-wave and work well. Both the TKB68 and TBK69 work with generic device handlers. I have a TKB69 on the tumble drier and I use a simple CoRE piston to notifies us by SMS when the it’s finished. If you want Samsung sockets and Curry’s are out of stock then try eBay. They’re not cheap but they are good quality and they repeat your zigbee mesh (note that the TKB will repeat z-wave).
Someone on here has integrated Lightwave (who produce replacement sockets) but its not straightforward and requires the other hub you want to avoid.
Have a look here: Wired wall socket with USB connectors
As for light switches there a number of options.I personally use Hue in high traffic areas with motion and door sensors as well as in the lounge with motion buttons and sensors. I’ve got a couple of Hue Tap’s where I need them. It’s a good clean solution but yes, requires another hub.
Good luck.