Since you have neutrals at the switchbox you can use any of the certified Z wave switches which are not labeled as “scene controllers” (although “central scene controllers” Will probably work with SmartThings, but will likely need custom code).
Sorry, I know that’s technical and confusing, but just ask about any individual model in the forums and someone will be able to help you figure out whether it will work with SmartThings.
Meanwhile, depending on your exact wiring, you might be able to use the following hardwired dual switch. You would use one switch as normal for your existing nonnetworked device, and hopefully use the other switch with the load tied off as a virtual control of your smart bulbs. What type of configuration is allowed in the US, I just don’t know for sure if it is allowed in the UK.