It would be easy to ascertain the purity of the metal. Trim or file a few grams (around 20g) off and send it by post to a scrap buyers, for example Cookson Gold, think they are in Brum. I used a similar service some years ago, I even got sent a secure envelope to post the metal. You will get the value of the gold put in your bank account and a report on what it actually IS, ie 9 carat, 18 carat. You may need to open an account.
I am not sure what things are like now but ten years ago I sent 1.3kg of silver cutlery for scrap to a place in London and got paid by return, no questions about where it came from. Jewellers and metal smiths often send scrap or unrepairable jewellery to get money back, you would be amazed at how much there is out there. It is not unusual for dealers/jewellers to sit on scrap gold for a while and wait for the price to go up, that is what bullion dealers do, they accumulate large amounts and sell it on when the price is right.
Declaring the stuff on arrival in the UK is up to the OP, as it's a gift or family heirloom I would not bother, it's not a prohibited item as far as I know. I would cut it into smaller chunks and if asked say it's scrap metal, which it is.
I've been out of the game for some time now so things have possibly changed, but the dealers are still there buying and selling.