...and, perhaps the most obviously distinguishing visual feature, the Lounge has a panoramic glass roof.
But things aren't quite that simple, since the original purchaser could have specified most (but not all) of the lounge features as optional extras on the Pop version - you'll find quite a few 500 pop's with A/C (back in the day, it was an extra £500 if specified when ordering). You could also specify, for example, Pop seats in a Lounge (that one was a no-cost option, and the Pop seats are much easier to keep clean).
Mechanically and performance wise, they're identical (as is the Sport).
The Cult, again, is just a different trim level - but be aware that the specific Cult trim parts will cost an arm and both legs if you ever need to replace them. (this is true for most of the special editions).
If you're buying an 8yr old car, some might say the Pop has less to go wrong (Blue&Me failures are commonly reported here, and the A/C condenser is also a known weakness (I'll bet an 8yr old car won't have many of the fins left intact). And if you break that glass roof (not common, but it does happen), it'll cost well north of £1000 to replace it (and many insurance policies exclude it from the glass cover section).
Some insurers put the Lounge a group higher; if you're young with no no claims history, that could be a factor (but for most, it won't significantly affect the premium).
On a car that old, I'd focus more on getting the soundest one you can find, with a decent service history, than on what colour it is or what additional extras it might or might not have.
