Translation:There is not good weather in the United Kingdom.
I believe not. In "Chan eil sìde mhath…", the adjective mhath is attributive (i.e. there is not a linking verb between sìde and mhath. In your translation you are using good as a predicative adjective, linked to weather by the phrase is not. Predicative adjectives are not lenited in Gaelic (unless lenition is caused in some other way, e.g. a modifier such as glè or ro). Of course, lenition of attributive adjectives only occurs with feminine nouns, so the situation might not be so clear if sìde were replaced by a masculine noun.