What is the most authentic source for considering exchange rate while preparing Annual Accounts?
I use xe.com
Xe.com or Oanda.
Spot or average?
I have tended to use Whillans over the years who I believe take their figures from HMRC.
I use x-rates.com.
Note the proximity of the letters 's' and 'd' on the keyboard. A simple typo could take you to a very different kind of website (I imagine).
Bank of England website
I use Oanda, but none of these sites is better than any other, and the Bank of England is honest enough to say so.