Anything larger than 2ml shouldn’t be sold in the UK given the TPD restrictions.
The TPD (which should be revoked ASAP!) instated a 2ml tank capacity limit. Your vape shop is correct. I’m not sure TPD (now TRPR) is being that rigidly enforced though and all my tanks have bubble glass inserts to increase capacity to 5ml. This obviously isn’t useful for disposables.
The nicotine strength of an e-liquid may be displayed as either mg/ml (or just mg), or as a percentage. Mg/ml shows the amount of nicotine per millilitre of liquid, and a percentage figure shows the amount of nicotine as a percentage per total. These measurements are the same across e-liquids and cartomizer refills.
A 12mg e-liquid is the same as 1.2% – this would contain 12mg of nicotine per millilitre of liquid. 1.2% of the liquid is nicotine.
A 3mg e-liquid is the same as 0.3%
A 20mg e-liquid is the same as 2%