You aren't necessarily - if you read through the application form it says that if you cannot provide 6 items then you must explain why not.
Basically, as soon as you arrive in the UK, get your name added to as many bills etc. as you can, register with the NHS, try to open a bank account (though you will need proof of UK address for this) etc. and save everything you receive in the mail.
Then, just provide as much as you can for the FLR(M) application and if you don't have 6 items in each name, you can explain why not.
Also, you will need proof of UK address in order to give notice to marry, so you'll likely have to wait until you have some mail in your name before you can give notice and then you will need to wait at least 28 days before you can marry (potentially up to 70 days if they refer your case to UKVI. though if you have a fiancé visa you shouldn't have to wait that long)
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