Change IP address

Short version: you don't, but that's likely not the issue.

Long version:

First, I assume you're talking about a wi-fi connection, since cellular connections are managed by your cell service provider, and it's extremely unlikely that they would give you an IP address out of London if you're not there.

If that's the case, then the IP address on WiFi is determined by the settings of your WiFi router, most often coupled with configuration data from your ISP. If your phone is using 'Automatic' or DHCP (check Settings -> WiFi -> <connection> -> ℹ︎ ) then your upstream device/provider is dictating your IP address.

However, all of that is still mostly irrelevant. IP addresses do not 'exist' in any specific location. They may be associated with a location, but that's just an arbitrary link where the IP address owner (i.e. typically your ISP) publishes data that says where any given IP address block 'lives'.

In this case you may be using an IP address that *used* to be reported in London and the ISP has reallocated the address to some new location, but hasn't updated the registry (or, more commonly, the commercial databases for IP geolocation haven't yet been updated, or a site you're using is using an outdated database).

Another alternative is that you're using a VPN connection. One of the effects of a VPN is that it masks your device's IP address and makes it look like you're somewhere else (e.g. the VPN server). Sometimes this is intentional. Sometimes not.

If you're using a VPN then that's largely part of how they work and to get proper geolocation you'll need to turn off the VPN connection.

Ultimately, though, without more data it's impossible to tell quite what's causing your geolocation issues. If it's a VPN, turn it off, anything else typically needs to go to your service provider.