Doesn't really mean anything. Even if the flat has a working phone line, they'll still charge you the installation cost. There's more to it than just having a working phone line - it has to be routed properly within the telephone exchange for ADSL to work over it which is what the installation charge will cover.
The same applies to Virgin Media - even if you've got your house wired up correctly and fibre optic cables laid on your street they still have to provision a modem for your account and set your service up at their end, and they'll still insist on coming round to ensure it's all done correctly.
Plus I'm sure Virgin Media charge for installation too, unless there's an offer on at the minute?
Whatever broadband type the previous occupants were using is kind of irrelevant. You're probably still going to have to pay for the installation whichever option you choose. Personally I'd go for the cable though, it's superior to ADSL in just about every way.