European Travel with "Essentials" plan. Purchase service when I arrive or do an E-Sim from a "local" carrier?

Hello to all!

So, I have a rough idea as to what I need to do, but I wanted to check with to see you all had any other suggestions.

According to the T-Mobile website, when I arrive in country, I'll simply need to log into the T-Mobile App and purchase a plan to grant access to data services and calling in the UK and mainland Europe. Sure it's won't be "cheap" but that's okay.

Is it really that simple? No catches?

Or should I purchase a one time single use prepaid E-Sim from a carrier to cover the week I'm in Europe between the UK and France?

Thanks for any suggestions!