"If Coulson has been found guilty yesterday, why should he be entitled to freedom on bail until he is sentenced ?"
The Bail Act of 1976 is quite clear. Defendants have a right to bail unless it can be shown that there is a substantial risk of them either:
absconding
committing further offences
or interfering with the course of justice.
Mr Coulson has been on unconditional bail throughout his trial, so presumably no risks as outlined above were raised. Although now he has been convicted he no longer has the right to bail under the 1976 Act, it is customary for bail to be extended until sentencing even when an immediate custodial sentence seems most likely, providing the conviction does not alter those risks. The most likely risk to be raised now would be one of absconding. Presumably the prosecution raised no objection to bail being extended (or if they did, the judge ruled against them).
It actually makes no difference because if he had been remanded in custody until sentencing that time would be seen as part of his sentence to be served.