Let us keep in mind that the purpose of my project (see OP) was to grow portulaca. Do people grow portulaca in the UK and other moist places?

Portulaca is a low-growing, heat-tolerant, drought-resistant plant whose flowers are remarkable for their pure and glowing colors. Its name has to do with 'portal,' or 'doorway,' and the ancient Romans planted it in the hard-packed soil around their doors.

One day, years ago, I was talking with a young, non-gardening neighbor. She was amazed that she could see a red bloom of portulaca, only one inch across, blooming in my garden from across the street.

(One drawback - it blooms in the morning. If you sleep till noon, you miss it. It also doesn't bloom on a cloudy day. It is a sunlover.)

My mother told me that it used to grow in the cracks of the sidewalks in Canton, Ohio when she was a little girl. I visualize Heaven as a place where portulaca does that.