You MUST have a "LUL entry permit" as a minimum to work on London Underground stations,
and it depends on what station it is, to whether or not you also need PTS.
IF it is a LUL owned station like FARRINGDON you just need an LUL entry permit, but would be unable to do ANY PTS work on that staion.
other stations like LONDON BRIDGE, you need PTS as well,
as this station is onwed by NETWORK RAIL not LU
so just to be safe and not get turned down for work,
its better to have both PTS and LU entry permit.
You must be sponsered by a company already working on the underground(LU entry permit) or mainline(PTS) to get both of these tickets.