|You cannot simply change your IP address to whichever you want.|
1. the IP address must be of the range of the IP addresses, you Provider has, so that you provider can router your internet traffic to the internet and back to you.
2. Providers do provide IP addresses automatically to the customers to and generally do not allow their customers, to use IP addresses they like to.
3. If you need an IP address of the UK to access special websites, you have to configure your browser, to use public proxy servers of the UK to get it to work.