How to pick the correct white?
I was hoping 1) to get some advice on the "gloss" or "sheen" issue (I don't even know what that means),


As well as the colour, the appearance of a painted wall is affected by how smooth and reflective the paint surface is.

Paints come in matt, silk, gloss and other degrees of shinyness. You probably want a matt finish as that is what a high proportion of British homes use on their walls. Note that people usually use gloss finish on woodwork.

You should expect to find matt emulsion on your walls, gloss paint is usually oil-based and intended for woodwork, not for painting on plastered masonry walls.


  2) again, how do people figure out which shade of white without having somebody physically there? 


By taking a sample to the store and asking them to match it. Stores often have a machine for mixing paints to an exact match. This can be needed because paint colours are fashion items and may become discontinued. Also, painted colours can change over time due to exposure to sunlight and other influences. Unfortunately, taking a chunk of your wall to a store is impractical. You may be able to scrape off a sample if the paint, or a layer underneath, hasn't fully adhered. A chip roughly one inch square will be enough.

or

By buying test pots until you find an acceptable match.