My ex wife is a kiwi. Uk passsport was being renewed. Took ages and she travels a lot. Just come in the uk line hand over the passport and the photocopy and explain. They then do a few more checks and let you in. Dot mess with the other passport as if you don't stamp out overstayer etc. They were happy on multiple occasions with the copy and there is always the backup of the overseas passport if you meet someone who is having a bad day. She had one and that persons co worker ended up saying from the next desk it is fine just let her in