I’m totally lost on e Sims. Please help!

I’m American traveling to EU/UK (itinerary below), and I need an e SIM, I guess? I’ve always just paid the $100 to AT&T when I go abroad, but I keep seeing that e SIMs are the way. I’ve read a lot about them, but I’m still lost. Would an e SIM or the international plan be better for this trip? How do you determine how many gigs you need? Do you purchase GB per country? I suppose I can live without iMessage or phone calls since we use WhatsApp anyway…

London : 30 April - 2 May

Eurostar >

Paris : 2 May - 5 May

Flight >

Florence, Livorno, Rome : 5 May - 14 May

15 days total, 3 countries