What a lovely pressie! It might be worth trying W&W Jewellery on Webbs Road. I did a fab jewellery workshop with them some months back and they had a whole range of gorgeous pieces at different price points including a pair of stunning diamond earrings for £400 ish. They may well have some pendant options for you to look at. Hatton Garden wise, I have used Eric Ross who was recommended to my husband when he asked various friends. For something contemporary but not eye wateringly expensive there’s London Road Jewellery, run by a local dad. What a lovely thing to shop for. Have fun!