Why does the Bangladesh Awami League need a UK chapter? I mean, what
does BAL, as a political party, have to do with the UK?
The U.K. has many immigrants from Bangladesh due to historic ties between pre-partition India (which was a British colony) and Britain. The economic and family ties fostered in that era persisted after Pakistan and India were partitioned and after the split of Pakistan into the modern states of Pakistan and Bangladesh.
Many of those immigrants continue to care about politics in their ancestral homeland. Some immigrants who care about Bangladeshi politics support the Awami League. A U.K. chapter facilitated their ability to support this political movement and to be kept abreast of its activities from their new homes in the U.K.
Are they an AIPAC-like pressure group working in the UK to secure BAL
or its leader, Sheikh Hasina's, interest in the UK?
Not really. It is more about facilitating Bangladeshi expatriate involvement in the politics of Bangladesh.
It isn't a foreign policy arm of the Awami League or a government in exile. Any involvement in U.K. politics on behalf of the Awami League is purely incidental to the chapter's primary purpose of allowing people in the U.K. to support and be informed about the activities of the party in Bangladesh.
What are their functions which cannot be provided from Bangladesh?
Allowing people with ties to Bangladesh who live in the U.K. to remain involved in and informed about Bangladeshi politics. This is naturally, much harder to do from Bangladesh than it is from the U.K.
U.S. Foreign Branches Of Political Parties Compared
In the same way, in the United States, both the Democratic Party and the Republican Party maintain some organization among U.S. expatriates who can still participate in those party's intraparty Presidential nominating process even though they often can't vote in the actual Presidential election in the U.S. since they have a non-U.S. domicile.