As it is pot grown, you can easily leave the plant in its pot for a few days/weeks before planting it. Just ensure you keep the plant well-watered, and it gets enough sun. Don't leave it for too long, however, as you don't want the roots becoming root bound.
The Lavender 'Hidcote' is very easy to look after, and the key to getting the best out of your plant, is pruning. Though you will need to carry out regular deadheading throughout growing season (to encourage flowers to produce), a thorough prune will need to be done as soon as any flowers start to disappear in August- we do recommend that you remove about 1/3 of the plant, as this will enable light to flow through into the heart of the plant, and which small, new shoots will then grow at least 5cm before the dormant season (in September).