Sorry, I really don't know.
At a simple level, getting BT to move the number first won't work because that will cease your Plusnet broadband. You would not be able to do a Plusnet house move for that reason.
At a complex level, perhaps it is possible for BT to place the order for a SIM (Simultaneous Provide) order for the phone line/number and a fresh provide of BT Infinity there, with you then literally cancellng the existing service completely. That is fraught with high risk, and depending on whether or not you are in a minimum term with Plusnet could be expensive. (Maybe not expensive in the overall context of a house move).
At a practical level, bite the bullet and migrate the fibre here and now to BT then get them to do the house and number move. I would think that is the safest if it is vital to keep the phone number.
Or put up with losing the number. Get a line and fibre put in by Plusnet on a house move and ditch the BT line.
In some of the above, split the fibre and line at the end of the new minimum term you would be on, if you feel it preferable.
There may be flaws in all the above, particularly given Plusnet's ability to mess up anything and seeming inability to do SIM provides within their computer system, needing manual control. But others may be able to suggest variations on one or more of those possibilities and end up with a viable plan.
Are you able to be in control of the line and broadband at both premises for a month or so?
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