The UK signed a free trade agreement with Ukraine last year.
You can find comprehensive guidance on how to import goods from and export to Ukraine here:
Trade with Ukraine
If you are physically exporting software goods from the UK to Ukraine, you can consult the online tool which provides information on tax and duty rates for a wide range of goods.
Check duties and customs procedures for exporting goods
You can also consult this handy step by step guide to exporting:
Export goods from the UK: step by step