Quick trip to London


Hi there, 

I see a few posts about this but I am completely confused. I will be in London for five days. I don't want to purchase an Esim, I don't even know what the sim thing is. I'm completely not tech savvy. All I want to know is if I have my phone not on wifi and a family member tries to call me via an app, such as Whatsapp will I get charged roaming? Will it not work at all?

Can I make and receive calls within a free app and if so what charges am I facing.