...
You should raise the invoice in euros - this will ask you for the
current exchange rate. Note that this is how many GBP 1 EUR will
purchase (not the other way around). On today's rates this is about
0.93.
Then when you apply the payment of the invoice (via Cash->Receipt) the
currency should be set to EUR and the exchange rate on the day of
payment should be given.
This is the procedure which I follow (rightly or wrongly!) and it
seems to
work most of the time with the exception of a couple of rounding
oddities
(which do not seem to affect the accounts, just some presentation).