I use a spreadsheet to arrive at the figures for our tax return, so it is fairly simple to change the rates for foreign exchanges. All our UK income stays there, so I have no actual figures to use.
This year I made one calculation at a £ to € rate of 1.138 and another using the daily rates from www.exchangerates.org.uk , which is quite compact to print out.
I always keep a copy and a note of the source for my figures, in case of any dispute.
The amount of tax payable was slightly lower using the daily rates, and more importantly, just below the next tax band applying to us, so I used those results for our return.
This also has the advantage of probably being a more acceptable source than second hand information that is available.