To understand the answer here, one must first understand how the London Stock Exchange categorizes securities. According to the Guide to the Trading System:
From a business perspective an individual instrument is assigned to a grouping known as a trading sector. A collection of trading sectors are grouped together to form a trading segment. A specific Trading Service is a number of trading segments that share the same market model.
In other words, under the London Stock Exchange, there are several Trading Services. Under each Trading Service, there are many Trading Segments. Under each Trading Segment, there are many Trading Sectors. Under each Trading Sector, there are securities. Each security belongs to one Trading Sector.
The Trading Services differ by market model (e.g. order-driven, quote-driven, periodic call auctions, etc.). Examples of Trading Services:
SETS — An order-driven Trading Service used for liquid securities.
SEAQ — A quote-driven Trading Service used for fixed income securities.
SETSqx — Quote-driven for some securities. Conducts 5 call auctions per trading day. Used for less liquid securities (e.g. those listed on AIM).
European Quoting Service (EQS) — A quote-driven Trading Service.
- EQS is for trading securities that are listed on other EU Regulated Markets, other EU Multilateral Trading Facilities (MTF), and Swiss exchanges. From my understanding, EQS came into being after EU regulations (MiFID) provided the framework to allow securities to trade on stock exchanges where they are not listed (similar to the situation in the US where NASDAQ-listed stocks can be traded on the NYSE, and NYSE-listed stocks can be traded on the NASDAQ).
- EQS is only for liquid securities. European securities considered non-liquid are instead traded on the SETSqx Trading Service.
Trading Segments and Trading Sectors
Examples of Trading Segments include "Exchange Traded Products" ("ETCS"), which is part of the "SETS" Trading Service. This "ETCS" Trading Segment is further divided into many Trading Sectors, including Trading Sectors for exchange-traded funds, exchange-traded notes, exchange-traded commodities, etc.
Securities are categorized into Trading Segments and Trading Sectors because securities differ in how they are traded (e.g. maximum permissible spread by market makers, place of settlement, allowed order types [iceberg orders, stop orders, etc.], etc.). The list of all Trading Services, Trading Segments, and Trading Sectors is provided in "Millennium Exchange and TRADEcho Business Parameters".
Now that we have the necessary London Stock Exchange vocabulary, we are able to give an answer to the question.
What is the meaning of ticker symbols that start with 0 (zero) on the London Stock Exchange?
Unfortunately, I have not been able to find an authoritative reference from the London Stock Exchange's website. From observations, tickers† that begin with zero are:
All securities under the European Quoting Service (EQS).
All non-liquid European securities not in EQS that end up under SETSqx instead. Specifically, these are securities categorized under Trading Segment "SSX4" and Trading Sector "SXEL".
All securities under the Global Equity Segment (GES) of the "SETS" Trading Service.
GES was launched in 2019. Foreign stocks under GES are traded on "SETS" and settled in CREST (the UK's central securities depository) in form of CREST Depository Interests (CDI).
Examples using the tickers mentioned in the OP's question and in the comments:
- Prosus NV (symbol: 0A28) — This is a security under EQS.
- Vanguard FTSE All-World ex-US ETF (symbol: 0LMO) — This is a security under SETSqx SSX4 SXEL. For some reason, the "country of share register" for this security is Germany.
- MICROSOFT ORD (CDI) (symbol: 0QYP) — This is a security under GES.
† In the London Stock Exchange's documents, ticker symbols (e.g. TSCO, BARC, HSBA, etc.) are known as "Tradable Instrument Display Mnemonics" (TIDM).