Just do the listing with UK postage and opt in to the GSP. As long as the tracking number is added after you post it and it is posted within your listing time frame, it will be ok. GSP don't always update the tracking straight away when they receive it but as long as the tracking shows delivery to the hub, your job is done.
I have never had an issue if the delivery has been a day or 2 later than shown.
Just make sure you pack it extra well.