You'd need to provide proof to HR that you're in a long term relationship (there's quite a few things to this, but in short you need to have documentation like child adoption certificates or mortgage documents). Once that's done, you can apply for Service Family Accommodation (which is a house instead of a single room). As for having a say in where you're posted, this is something everyone gets, and the Army will try to post you somewhere you want to go, but that's not always possible (which is part of the job).
I hope that helps to answer your question, please feel welcome to post any follow-up questions in the comments.
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