Helena

I wish you all the best, the letter you asking for is a PR (personal reference), simply I will not to do all the wiriting for you, but I would give you some suggestion.
Firstly, a good PR is to tell the university you apply to that you are the right person they should give you an offer. As I have read from your question, I think you could say that you are an active student union member in your previous university and you are hard working,etc.
The second thing you should add in your PR is why you want to study finance and accounting.
The last thing I think you need to mention is that why have you chose this university, for example, this university could maxize my dream, etc.
Agian, wish you all the best and have good fun in England.