If you want to mail a letter in Great Britain, look for a pillar box. Whether in the form of the pillars for which they are named or set into walls (yet still called pillar boxes), they are always red. People have been dropping letters into them since they were introduced in the 1850s.
Pillar box designs change. The more modern ones are round or, if they have two slots (one for in town and the other for out of town), oval. This design dates from 1866-1879 and is the Penfold Hexagonal. You’ll find it on a residential street in Chiswick in West London.