Exchange rates go up and down, so it is always a gamble whether you will hit a rate that is better or worse. Ideally you want to change your pounds to euros about two years back...
There are a number of online sites where you can see a graph of how rates have varied over a period of time. You might be able to use one to decide if they are going in a direction that might give you a better rate if you wait a while. Alternatively things might be plummeting in a fashion that means jumping in quickly is a better bet. Whatever you decide, it will probably change the day after you make your purchase...

As for best deal - the last couple of times we found the local Thomas Cook gave a good rate compared with post office or banks. That was before their recent financial troubles.

Quack!
GtD.