Nevermind... I called Three and got my questions answered. The "Get 3000 texts and 300 minutes plus 12GB data for just £20" sounded good to me.

They told me the the free Three SIMs are readily available at supermarkets and that I need to buy a top-up coupon, which I can buy with cash. And, they suggested I call them to make sure that it's applied towards the "Pay As You Go - All in One £20."

They confirmed "Feel At Home" is included in that. 12 gigs of data for the ~2.5 weeks I'm in Europe should be more than enough.

They said there are activation instructions on the back of the package.

Thanks for suggesting Three. The carrier and plan choices in the UK are overwhelming.