I don't believe your browser has any concept of location. At least not that you can change in any settings.
I believe most websites that determine your location do that based on your point-of-presence. That is, where you actually touch the Internet. For example, I live in southern California. When I'm at home, most web sites see me as being pretty close to my actual location. When I'm at work, most web sites see me as being in Dallas Texas. Even though I'm sitting in SoCal. The reason is that my company's POP for the Internet is in Texas.
So, if you're trying to make yourself look like you're coming from somewhere else, you need to find a proxy site in the location you want to be seen to be coming from. Or, maybe use something like a TOR browser since that tries to hide your location.
If this is some issue with getting the BBC iPlayer working, you might try contacting them at the iPlayer site. I don't know anything about iPlayer, but from a quick Google, it looks like it's a compiled app, so your browser shouldn't really come into play when using it. But, like I said, I don't know anything about it so maybe ask them at the iPlayer site.
