I don't agree with your statement regardless you are probably also Czech guy like I am. I really think that the only limitation are bands, thus Apple has three different models for three different parts of the world. So, if you buy watch in the UK, you can use it with domestic service in France or in Germany, for instance. Roaming is different story. It won't work because of duplicated phone number. Networks usually don't allow two SIM cards with the same number active at the same time, regardless you might have officially two SIMs (let say the first SIM in phone and the second in modem). You always have to maintain turned on only one of them at all times. Now you have 2 SIMs with the same number turned on concurrently. That would be a mess in roaming, especially in countries (carriers) which can't handle such situations, therefore roaming is blocked for Apple Watch SIM only. On top of that Apple Watch has to use VoLTE in 4G network (there is no other option for calls in 4G networks except CS Fallback). VoLTE itself doesn't support roaming. UMTS networks have way worse coverage not only in Czech Republic because of the frequency used, so it can't be used as the primary way for AW calls.