Whoever added the UK PSN listings for Timesplitters has screwed up. If you search by title, it looks like only the first game is available. You need to go into the product page for that one and bring up the list of editions to find the sequels. This is the same nonsense that happened when R-Type Final 3 was listed as an edition of Final 2, an error that has never been fixed.
The good news is this shouldn't stop you buying any of them; it's just going to mean that a lot of people won't know that all three games are available.