They don't charge any commission on foreign currency and only 1% on sterling travellers' cheques. I went to the M & S outlet at Arena park near my house and got €245 for £200.36 (@ rate of 1.2228).
A lot of people tend to change currency in banks but ore often than not, their rates are not the best. Doing some research, I learnt that the worst rates are those offered by those money changers operating from small kiosks.
I too did make some mistakes on the way and bought travellers cheques worth £500 from the post office a few days before that. They charged me £7 pounds as commission.
If you are interested in