There are many solutions, and the only reliable advice is, do not use a High Street bank!
I used to use FC Exchange, call them at the telephone, get a quote, agree it, transfer the money in £ and leave them to transfer the €.
My preference now is to use XE Money Transfer at https://www.xe.com
On the first page you can type in the amount and the two currencies to find out the current live rate.
Right now, the rate is about 1.18.
I say 'about' because it depends on the amount being exchanged: the greater the amount the better the rate.
Right now it is quite volatile with news of Brexshit, and the results of election polls, having quite a dramatic effect.
On Friday, the rate was 1.16, by Monday, when I had to make a transfer, it had dropped to 1.14 and, today, it is up to 1.18.
With a transfer of £10,000, that resulted in a loss of €400.
I am afraid that my crystal ball is no better than yours.
With XE, all fees are included in the quoted rate, and when you compare rates with other providers you should check that the rate is inclusive.
This does not mean that there are no charges.
The charges are built into the rate.
Your question should be, if I give you £100 now, including all charges and fees, how much will I get in €?
You also need to allow at least 1 day for an international transfer and, if you miss a cut off deadline, this can take an additional day.