The feature you are looking for may be more custom the Shopify store has to offer for free so you may need to build a custom private Shopify app to make this happen. If you need me to set this up feel free to email me at LMPTechSolutions@gmail.com
Hi @rosejay - is this all sorted? If you are on a look out still and want to partner with a UK Charity - look at Pledger - UK Giving App https://apps.shopify.com/pledger. Its built for this exact user case. Your donations automatically go to your chosen Charity.