20mg Nicotine
20mg is the strongest e-liquid that can legally be sold in the UK, and it is the strength most people need when they first stop smoking.
If you were on twenty a day, start here. Anything lighter and the cravings tend to win - the most common reason a switch fails is starting too low and going back to cigarettes because vaping did not satisfy.
Almost everything at this strength is a nic salt. Salt nicotine is absorbed faster and sits far smoother in the throat than freebase at the same number, which is what makes 20mg tolerable at all. Use it in a pod kit with a coil of 0.8 ohm or higher.
Once you have been off the cigarettes a while, 10mg is the natural next step down.