You can donate money to the many organisations that are supporting Ukraine.
The National Bank of Ukraine have set up an account that accepts multiple currencies, including sterling:
https://bank.gov.ua/en/news/all/natsionalniy-bank-vidkriv-spetsrahunok-dlya-zboru-koshtiv-na-potrebi-armiyi
There are many other organisations that are providing various types of aid, too: the Red Cross, UNICEF, many others (I don’t have links to hand, sorry).