Who legally has to leave the house?

If me and my boyfriend and our two young children buy a house but the mortgage is in his name only, who will have to move out if we separate? Will it be him as it is legally his house, or will it be me and the two children ( 3 and 11)? Only one of the children is his.
Would I have any rights whatsoever as we moved in together from the very start.
Thanks in advance.

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