For example, I read in a BBC article earlier about a Roman temple found in London that is seven metres below today's city.
How/why does this happen, and how long does it take before it becomes buried to the point of being forgotten?
This is really an archaeology question. Here's a great answer from /r/archaeology.
There are some regions where the ground level is falling due to erosion, and others where it’s rising due to soil deposition. The former regions tend to be barren, at higher elevations, and have little topsoil; the latter tend to be at lower elevations, near river valleys, and have rich soil. Humans have a strong preference for building their settlements in the latter sort of region, where the ground level rises over time.