i need an answer to below question, but i strongly urge you to first read below infromation to reduce misunderstanding of the subject
What is roaming
Types of roaming:
International roaming: Most common type, when you go abroad and make a call you use international roaming with prices calculated by the agreement of your provider with a local provider.
National Roaming: Its when you venture beyond strong signal of you parent network, your phone have then capability to switch to strongest local network, even if its not parent network, extra charges may well apply, depending on national agreements between service providers. National roaming works only within parent country (here it will be UK)
Data roaming: same as both of the above but relies to data; can be either national or international
Managment of roaming: you have full power to switch on/off both international and national roaming on your actual device, but in both cases your provider must enable it first in the network setting.