I am looking to limit registration for my app on whether they are from London.
I am not sure what is the best way to do this, I could use iOS' location API but not sure if this is easily hackable.
I use a PHP backend so could extract the location via GEO co-ordinates from the IP Address, but heard this could be unreliable.
Does anyone have any experience with this and what is the best/secure way to do this?
Your first idea (using the iOS Core Location API) is the best. Indeed, you cannot rely on IP for geolocation. When it works, it's not accurate. And, most of the time, it just gives a wrong location. And it's easily fakeable (with a VPN for example).
From my point of view, you have to rely on the iOS Core Location API. It's the most reliable solution (and the less fakeable). But, to be clear: I don't know any solution to be 100% sure of the user location in iOS. The only way would be to install some equipment in the targeted area (like iBeacons).