Ask UK

I'm not sure if you are taking about roaming in Europe or beyond. In Europe, its likely that your provider will not charge you extra, but still worth checking. To do this best to google the name of your provider + roaming and the name of the country you are visiting

Further abroad its very hit and miss. I recently travelled to a country where my provider charges £30/MB! In that instance, I instead used an app called "Airalo" and they provide virtual sims. Follow the instruction in the app, and make sure to only download the Sim over wifi if you are already abroad otherwise you'll be charged the roaming fee (as I learned the hard way). It worked flawlessly for me.

There's a breakdown of roaming charges by network on the MoneySavingExpert site - you might not actually need to do much.

In an odd twist of fate, a lot of the cheaper operators (e.g. iD mobile, Lycamobile, Giffgaff) still have free roaming in the EU+.

Which phone do you have? If your phone supports eSIM, it's a lot easier to get a local SIM card - especially in places like India where you might get scammed if you're not careful.