the phrase "bless up" is not inherently a British expression. It is more commonly associated with Caribbean English, particularly Jamaican Patois, which has had a significant influence on the multicultural aspects of British culture.
In Caribbean English, "bless up" is an expression of goodwill, encouragement, or gratitude. It's similar to saying "blessings" or "God bless you." It can be used to wish someone well, show appreciation, or offer support. For example, if someone is going through a challenging time, you might say "bless up" to offer them encouragement and positive vibes.
It's possible that the phrase "bless up" has gained popularity and crossed over to be used by people from various cultural backgrounds, including British people, due to the influence of Caribbean culture on British society.
However, language and slang can evolve over time, and new meanings or interpretations of phrases might emerge. It's always essential to consider the context and the specific individual using the expression to fully understand its intended meaning.