How do I report foreign dividend income in GBP that was automatically reinvested and how do I know what exchange rate to use?

Use the Foreign Income screen in Turbo Tax, end of the "T-slips" section. Foreign dividends are reported as simple Interest (investment) income. You can provide the information in GBP and add the currency conversion in top right corner as 1.7299 (annual) or make the conversion and then report in CDN. Leave currency conversion blank in that scenario.