I've ruined the wash basin overflow.

[polkadotpjs]

I did the taps with a penny and was amazed at the limescale coming off so well. So I did the overflow and totally removed the silver metal coating and now it looks like the white plastic I presume it is. Any fixes??!

Yes just get pull it off with your nails and buy a new one, just measure the diameter of the overflow.
YIGO Sink Hole Round Overflow Drain Cap, Type 2, 3 Pcs www.amazon.co.uk/dp/B01E7CQSFW/ref=cm_sw_r_cp_api_fabc_53lbGbQT96R5E?psc=1&_encoding=UTF8&tag=mumsnetforu03-21

Sounds like you've removed the chrome coating.

Buy a new one from eBay/Amazon. Shouldn't be too ££££