It'd depend on the actual programme format, availability of guests etc. Some of the shows I've worked on have been live, save for a few short pieces where they recorded an interview with a particular guest earlier that afternoon, as he or she wouldn't be available in the evening. Those sections are slotted in 'as live' but are actually from tapes. Others are recorded in the afternoon and broadcast that evening. One show we did, the majority of it was recorded one day, then because there was a phone-in element part of it was also broadcast live the next day. That was to give the edit suite time to cut down portions of it, bleep out swearing etc. and because it took a long time to film the actual body of the programme, pretty much all day in fact, just to get enough to then pare down to the juicy bits.
If there's any sort of competition phone-line or phone-in (especially at a premium rate, which most of them are) then nowadays they have to be live. (I have heard some stories of how they used to rig those and pre-record everything, but OFFCOM doesn't take too kindly to that these days).
