Wedding gift for fiancée - help!

Hi all! I'm looking to get my fiancee a diamond necklace as a wedding gift – we are getting married next month. My budget is £2-4/5k. I am currently thinking of getting her something from Tiffany, or possibly a non-diamond necklace from Cartier. This is all pretty new to me so any guidance would be appreciated! Are there any brands I should go for/any brands to avoid? In terms of style, I'm after something not overly ostentatious, a rather simple design. Probably sterling silver or platinum. TIA!!