Can you move into the house you are buying before completion?

I have asked the vendor if I can move into the house before our mortgage completes (he hasn't text me back yet so I am presuming he doesn't want to do it) but if he does, would this be ok? Our notice on the house we rent runs out on the 2nd August and we just want to move in now we are sick of waiting. Anyone any ideas on this? Thanks.

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