I also had a substantial amount of euros to exchange. I would recommend you use a currency exchange company such as World First or HiFx etc. as the rate they give is so much better than banks. They do not charge any commission and the rate stated is the full amount which is received, unlike banks. You can also request a "Market Watch" which means you state beforehand the rate which will be acceptable to you, and when/if it reaches this then you can sell very quickly. However, if the exchange rate is good enough on the day then you can have a Spot contract; it just depends on how well the euro stands up against sterling currently.
The sterling is then transferred immediately to your bank account which you can put into any savings account you wish. There are a few out there approx. 3% and above, most of them online such as Nationwide, AA, Santander etc. I know there are some above 4% if you want to stash away your money for 5 years but, due to what's happening at the moment, I personally would not even think of doing this long term!
Hope this helps, it was ideal and very simple to do and I got a rate of just over 1.15 - however that was last November and things have changed quite a bit since then.