Never change it all at once, apart from anything else you might not spend it all.
These days the rates not great so change maybe 200 quid at a time, the mall vendors are often in a little wooden cubicle with a glass front and you might not feel comfortable but they are almost always honest.
I've only ever suffered an attempted scam once at a money changierthat did not have a name on the front, looked respectable but wasn't, we got the Police on them and got our money back there and then had a police escort to a good money changer![]()
The scam was hand over X they take it into the back shop come back and say it was X-100I blew my top as did Ana
Jim