The contention ratio is a one aspect that influences the speed of your internet connection, and is particularly relevant to built up areas such as London. Formally it is the amount of people trying to access a bandwidth resource. This is represented as a ratio such as 100:1, meaning 100 people trying to access the same bandwidth.
For your home broadband this refers to the number of households sharing the bandwidth that your ISP has available. It’s better to have a lower ratio as this means that fewer people are sharing your ISP’s bandwidth. A fairly standard contention ratio is 50:1 for home broadband.
Let’s highlight this with an example. Let’s say you have a 50mbit home broadband line on a 50:1 ratio, and your ISP has leased capacity for your area of 50mbit (unlikely low), then assuming that only you are maxing out your line you will get the full 50mbit. However. If all 50 are maxing out their lines at the same time (again, unlikely), this reduces each household’s connection to 1mbit.
Now, this is an over simplification (and things like bandwidth shaping are applied by your ISP), but it demonstrates the principle.
Unfortunately it is not generally possible to determine the contention ratio for your ISP in your area, so most often you are working off rumour.