For passive voice, you are describing an activity with the object at the start of the sentence.
The salmon has been smoked - an activity: passive mood
When a past participle is used to describe the state of something, its usage is adjectival:
The salmon is smoked - a state: adjective
It's difficult to tell without a context, but your first three sentences probably describe a state, not an activity: for example, the state of the park is "covered with snow". If the context gave some clues about how the snow got there, it could instead be taken as an activity. When you use a past participle to describe a state, its role is adjectival.
The final sentence could be regarded as either describing an activity (people speak English in the UK) or, more likely, a state (English is the language that is spoken in the UK).