Match it to your smoking, not your caution. A pack-a-day habit maps to 20mg; ten a day sits closer to 10mg. The most common switching mistake is starting too weak and vaping constantly to compensate, which feels worse than one properly satisfying strength.
UK law caps nicotine e-liquid at 20mg/ml, full stop. Anything sold stronger is non-compliant grey imports. If 20mg genuinely does not hold your cravings, the answer is usually a better-matched device or nicotine pouches alongside, not an illegal bottle.
Salt, almost always. Freebase at 18 to 20mg is famously throat-scraping, which is exactly the problem salts were invented to solve. The freebase versions still exist for the minority who want that kick; everyone else should assume 20mg means salt.