Are lavender plug plants a good idea

If it were me I would pot them up . I find that with all plug plants it just gives them a better start.

I would advise you to pot them up for a couple of months at least. That way they will make bigger plants & have a decent root system when planted into the garden.

Pot them up but dont over water them.
They will sit and sulk if you do, its happened at the nursery were i work, they got rained on whenever it rained and haven't grown much if at all! normally they are fine if kept uinder cover- outside but away from heavy rains and watered by you

Thanks for the advice everyone, potting up it is!