The system used for local elections in London boroughs is known as "bloc voting".
Each local government district (ward) is represented by 2-3 councillors. If there are 3 seats to be filled then each voter is entitled to mark 'X' beside up to 3 candidates on the ballot paper. The 3 candidates with the most votes are then elected.
It is known as bloc voting because:
- The optimal strategy for a political party is to nominate as many
candidates as there are seats to be filled.
- Typically voters will cast all of their votes for the same party.
- Typically the same party will win all of the seats in a given
Wikipedia also refers to the system as "plurality at large" or the "multiple non-transferable vote". (https://en.wikipedia.org/wiki/Plurality-at-large_voting)
Bloc voting has all of the same flaws as the single-member district plurality system used for national elections in the US, UK and India.
It tends to produce highly disproportionate, semi-random results, and there is no guarantee that the party that wins the popular vote will actually be awarded the most seats in the government body being elected.
Under bloc voting the distortions of "first past the post" are actually aggravated because it is easier for a single party to win an unmerited landslide. And we see this in the many local councils in London that are dominated by massive single party super-majorities.
Where multi-member wards are used outside of London staggered elections may be used, with one member elected at a time, in different years.
Potential for electoral reform
Advocates of retaining "first past the post" (plurality) voting for national elections tend to place great emphasis on the importance of single member districts as a way of preserving a special link between representatives and their voters.
But this argument obviously does not apply to the multi-member district system used for most local elections in England, which FPTP advocates seem to be quite content with.
Because the same multi-member wards could be retained without any redrawing of boundaries, the current system could be quite easily replaced with superior methods like PR-STV, or the semi-proportional single non-transferable vote (SNTV) system.
SNTV has the added advantage that it is very simple to explain and implement, and would not require changing to a new ranked ballot format.
On the other hand PR-STV is already used for local elections in Scotland and Northern Ireland.