Obtaining sim for use in UK, Ireland and Paris

[jenverk]

I live in USA.  I am traveling to London, Paris and Ireland for 17 days total.  I am looking at purchasing a sim for use on my trip.   Can I stop in a Vodofone store and have someone switch out my sim and set up pay as you go plan right when I arrive? 

[BandOfBrothers]

Yes a Vodafone employee in a Highstreet Store will only be too happy to provide a pay as you go sim card and explain the best tariff and explain how to top up the credit on the account @jenverk 

https://www.vodafone.co.uk/mobile/best-sim-only-deals/pay-as-you-go-sim. 

As Vodafone networks are not linked in different countries they'll also explain what roaming costs would be for other countries and if you'd be best just picking up another local sim card when you travel.

Vodafone Uk > costs-and-charges/travelling-abroad 

Your phone will need to be unlocked to all networks to accept the sim cards.

Enjoy your trips.