I need your advice, please.

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From what I've heard about people like yourself (& myself who have boyfriends in the UK), bring as much supportive documentation as you can.

You already have a return ticket, bring along bank statements to show you have the funds to support yourself for 3 months & even a letter from your dad (saying he'd support you if needed) i.e. all of this is to show you won't be a "burden" & apply for government social funding or something. Even a letter from your boyfriend saying that you'll be staying with him & he can support you financially if needed & his work is .... & if you want to overkill it, have him bring his bank statements as well.