No onward travel plans
This is the most common issue. If officials see no evidence of your plans to leave, this will be a red flag. The best thing to do is to make sure you have evidence of your return flight.
Previous entries
If you have been detained by UK immigration officials before, this may result in you being held up again – even if you were approved the last time.
Returning to the UK shortly after leaving
There have been reports of non-visa nationals leaving and re-entering to renew this their months’ allowance. If you take short visits to Europe before returning to the UK, it could result in your travel patterns being flagged by immigration.
Periods of overstaying
If you have previously failed to leave the UK when your visa expired, this will cause you difficulty at customs. Periods of up to 28 days are not counted by the Home Office, so leaving as soon as possible after your assigned leave reduces the problem.