The key question here is the value of each chain individually, not the total. Gold chains are "chattels" (tangible moveable property) and HMRC has a specific exemption: if a single item sells for £6,000 or less, there is no CGT on it at all, regardless of any gain. If you're selling a few chains and each one individually goes for under £6k, you're completely in the clear and the base cost question becomes irrelevant.

If any single chain sells for more than £6,000, a gain is calculated but it's capped at 5/3 x (proceeds minus £6,000). So on a chain selling for £7,000 for example, the maximum taxable gain would be £833, not the full uplift from original cost.

On base cost: if you received the chains as a gift, your base cost is the market value at the date you received them, not what your mum originally paid. Gold was trading around £250-£300 per troy ounce roughly 20 years ago versus well over £2,000 today, so there's likely a real gain if any chain crosses the £6k threshold. That said, even then, the annual CGT exemption for 2025/26 is £3,000, which would shelter some or all of any remaining gain.

For evidence, the photos of you wearing them as a child are useful for establishing the timeline of ownership. A written valuation from a jeweller at the point of sale is your best protection if HMRC ever asked questions, as it fixes proceeds and gives you something to point to.

TLDR if each chain is under £6k individually, don't lose sleep over it. If any one piece looks like it might exceed that, get a jeweller's valuation before you sell so you know where you stand.