First of all, I love lavender! It is a classic of the plants world and quite rightly a staple of many peoples gardens!
Quintessentially you may see plump, billowing hedges of lavender or informally placed Lavender shrubs in a Cottage Garden, but growing Lavender in pots is a great way to add a touch of design flare to a contemporarily designed garden.
When it comes to growing Lavender in pots, always aim for a large sized pot. Something around 45 to 55cm in diameter should be a good fit. The reason for this is that Lavenders are very good growers and will require, and benefit from the extra space.
Also bear in mind the type of conditions Lavender would be found growing in their native habitat in the mediterranean. Naturally Lavender would be found growing in sunny, hot, dry conditions, typically getting around 8 hours of sun per day. The silver coloured leaf is a good giveaway here; you will find that any plants with this characteristic will require full sun to really thrive.
Therefore when it comes to potting up your Lavender you to firstly make sure your container has plenty of holes in the bottom and either some gravel, pebbles, or the old favourite, pieces of broken terracotta at the bottom to provide good drainage.
For the soil, you want to make sure this is free draining. You can do this by mixing coarse sand, or gravel with John Innes 2 (or an equivalent medium).
A 20:80 ration here would be good.
Now the type of Lavender. I was asked this because my client (and a few others I'm aware of) has been having trouble growing Lavandula stoechas, French Lavender, in their garden of late. It seems the very mild, wet winters we are getting are causing a bit of an issue with types of Lavender currently. My assumption is that they are having to endure long spells sat in moist, poorly drained ground and unlike the stronger English Lavender, are giving up the ghost.
My advice if you want a greater chance of your Lavender not succumbing to our nasty, wet winter's is to go for something like Lavendula angustifolia 'Hidecote', or 'Munstead', exceptional English Lavenders.
However, if you make sure to follow the above guidelines when potting up your Lavender containers, you shouldn't have much trouble at all growing any of the wonderful French Lavender such as Lavendula stoechas or Lavendula dentata.
Hope this helps!