Entering back into the UK on a new passport?

[Ginjarou]

Hi all,

Have been trying to find out about this but not such luck.

My partner is on the 5 year route. In December we will apply for ILR having completed the 5 years.

In May we're going to visit her family in Japan but he passport expires in January 2020. I understand you need 6 months left on your passport and then you should renew it. We're wondering if she can leave the UK on her current passport, apply and receive her new passport in Japan and then return on that? I assume she can just keep the current one (though I have no idea how Japan does things).

Anyone know if this is okay / has any advice?

Thank you.