Have you had a chance to look at the UK lighting FAQ yet? I think it will answer most of your questions.
Although there aren’t any listed on the official “works with SmartThings” page for the UK, both the Aeotec and Fibaro brands are popular with UK users, and the Fibaro can be used without a neutral in many cases.
@anon36505037 is in the UK and has done his whole house with Fibaro kit, so he may be able to say more.