Why have seasons 1 and 2 been taken off free to watch?

I did see someone else saying series 2 wasn't now available and when I checked series 1 was still available, pretty sure that was earlier today. Just checked again and you are correct series 1 isn't now available, these things are controlled by Sky I believe so Virgin can't just add it back. Fortunately I decided to watch them after seeing the series 4 adverts. I would suggest watching the the series 3 + 4 series while available and then wait for series 1 + 2 to become available again they sometimes do. As the series don't follow on you are not going to miss any continuity. You might need to wait for the best series to be available again but that's down an individual's choice.

All VoD programmes have their planned expiry date listed on the V6 EPG, there's a panel on the programme listing which states "available until".

Available content does come & go - generally series should be available in their entirety but not necessarily every series.

S1+2 are still available on Sky and NowTV, the removal from Virgin might be a mistake?