You cannot smoke and buy tobacco until your 18
You can smoke and possess tobacco products at any age in the UK.
It is illegal to sell tobacco products to a person under 18 in all the countries of the UK. If a person under 18 succeeds in buying tobacco products in England, Wales, or Northern Ireland, the offence is committed by the seller, not the buyer. In Scotland, both are committing an offence.
It is currently legal (November 2014) for a person over 18 to buy tobacco for a person under 18 in England, Wales, and Northern Ireland, but illegal in Scotland. A nationwide ban on proxy purchases is expected soon.