Listing and delisting information for stocks
Thomson Reuters Datastream has a datatype Basedate (BDATE), which is the date from which Datastream holds data for an equity series. This will either be the listing date, or the date when Datastream started covering a market (1964 for UK).
Datastream has a datatype Time (TIME), which is the latest date for which it holds data. This will either be the delisting date, or the current date for active equities.
To get all UK companies that had a listing at sometime during 2006, you could take the constituent lists FBRIT (Active) and DEADUK (Dead) and select those equities with BDATE 31/12/2005 (active during 2006).