step child name change on BRP

[tslondon]

Hi

i hope someone here has some experience of this.
I can not find any similar posts, so am reaching out for help.

situation:

My wife changed here surname on her passport and BRP, so no problems here.
My step daughter turned 18 this year and applied for a new passport (Russian) , but at the same time also changed her surname to be the same as me and her mother.
SO we all have the same family name.

She has applied for a change of name on her BRP and this is in the system being processed, but we dont know how long this might take.

So I think we have everything covered, but the question is if we need to travel before the new BRP arrives.

We have travel arranged in early Jan.
I know leaving the UK is ok, but when returning we need a person in another country to understand the situation and allow boarding.
At the UK border they can check her bio-metrics if needed.

What she will have is Her old passport and a BRP that matches that name and then she will have a new passport that matches her ticket. She may not have a BRP that matches her new passport name.

Ive seen that a wife can carry her marriage cert, but what can a step child show to prove that they have changed their name also as a result of parents marriage.

Common sense would say that if we all travel together and my wife takes the marriage cert, then there enough there to show the story, but I fear that common sense is not a legal definition.

I was wondering if we need to get something in the UK notorised that can show she is the same person.

thanks for any advice or similar experience