In most types of employment you're entitled to a single 20 minute break if you work for more than 6 hours in a shift. (The entitlement is neither 'cumulative' nor 'pro rata'. You don't get any more breaks, or longer ones, for longer shifts). During that break, you must be permitted to leave the business premises (e.g. for a smoke) if you wish to do so.
There are certain exceptions to the regulations, particularly if you work in the security or rail industries (or in other environments which require continuous staffing, such as hospitals).
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10029451
If you work in enclosed premises, your employer must prohibit smoking. (If the employer permitted you to smoke, both you and the employer could be fined). Many employers now also prohibit smoking other than in enclosed areas. While there's no obligation for them to do so (except where safety considerations apply, such as in the oil or plastics industries), they're perfectly entitled to ban smoking throughout their premises. (There is no obligation upon employers to make any arrangements for smokers. It's illegal to provide 'smoking rooms').
Give it up!
http://www.quit.org.uk/
Chris