**Try a specialist postal exchange service.** I found two that explicitly accept Zambian kwacha:

- **[Leftover Currency](https://www.leftovercurrency.com/exchange/zambian-kwacha/)** accepts current and older Zambian banknotes. Select your exact notes on its website to see the payout, then post them in. It offers payment by bank transfer, PayPal or cheque within five working days of receipt. [How it works](https://www.leftovercurrency.com/how-it-works/).
- **[We Buy All Currency](https://www.webuyallcurrency.com/exchange/zambian-kwacha/)** also advertises acceptance. Its listed current-banknote rate is £0.024208 per kwacha—**about £29.05 for K1,200**, subject to its final valuation.

Check the **pictures and series of your notes** carefully: Leftover Currency lists substantially different payouts for different issues. Compare the actual quote and postage cost before sending anything.

Since this was a work trip, another practical option is to sell the cash to a colleague heading to Zambia at an agreed rate. For this amount, that could give you a better return without postage costs.