Excel Formula

Hi all!

Firstly, dropbox link is here for this query! - https://www.dropbox.com/scl/fi/za56sfam3g1h4no6vzesr/Excel-Formula-Query-Dropbox.xlsx?rlkey=10ge3j2jjnpky2cjok5zojr1q&dl=0

I'm very unsure what formula to place on this spreadsheet of mine, we're using this to track the spend on travel and subsequently there are different exchange rates displayed. I'm trying to find a formula to use which will read what was manually input by whoever fills this sheet in (If they select GBP/EUR etc) and then convert to Local currency (In this case, GBP) - I have provided a table of rates, but just don't know what the best 'One formula fits all' is! 

Your help is much appreciated! 

Thanks,

Will