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I met my boyfriend on an online gaming site and it was love at first site, even though we haven't met in person yet. [...] He has asked me to move there after Christmas and marry him which I plan on doing.
One of the requirements of getting a fiance(e) visa is that you have previously met in person. If you haven't met in person yet, you will not be able to get a fiance(e) visa.
So, at this stage, your options are to meet up in person and gather evidence you have done so (ex. photos of you together) before applying for a fiance(e) visa or him to come to the US, get married, and then you can apply for a spouse visa.
ETA: Someone may step in who knows a bit more about the MVV, but I think that may also be an option to allow you to go the UK, get married, then come back to the US to apply for a spouse visa. I don't know off the top of my head if it has the same requirement that you have previously met in person, but if not, that may be another option for you. To note though - you cannot stay in the UK after getting married with a MVV. You must return to your home country and if you want to come back, apply for the appropriate visa.