Ask UK

If in Europe, if you don't have included roaming, most MNOs offer roaming add-ons. Outside of Europe, better value is to pick up a local SIM on arrival, and buy a bunch of data. What I do when travelling outside of Europe with family, is that I take a mobile hotspot and put a local SIM into that, and then allow the rest of the family to connect to that. The advantage of this is that you control the connection - everybody else has their phones set to no roaming (and therefore no risk of uncontrolled cost), and you can topup as required. I have a NetGear AirCard - has a swappable battery so you can keep one spare fully charged.

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+.

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

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.

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.