In the UK a life sentence does mean life. When the sentence is given the judge sets a 'tariff' which is the minimum period the offender must serve in prison before they can be CONSIDERED for parole. Once this period has passed (and assuming further time has not been added for 'bad behaviour', etc.) then the prisoner is eligible to apply for consideration of release on licence to the parole board.
The parole board consider this application and take evidence from numerous sources as to the prisoner's suitability to be released back into the community. They will consider such things as the risk this person poses, both to individuals and society generally, whether the issues which led to the offending behaviour have been addressed (e.g. anger management courses), signs of acceptance and remorse, etc.
If the parole is refused, the offender must wait a certain period before they can apply again, and would be advised to address the issues raised by the board as their reason for refusal. If parole is granted the offender is released on licence - which remains for the rest of their life: hence it is a life sentence.
When someone is on licence they are subject to recall to prison AT ANY TIME, regardless of whether they have committed a further offence or not, if their behaviour, based on the offence for which they were sentenced, shows'cause for concern', or if they breach any licence conditions which have been imposed. Such 'breach' proceedings are generally brought by the Probation Service (working closely with the police) and involve a further court hearing at which point the offender may be subject to immediate recall to prison and the whole cycle starts again.
The biggest problem with this system is it requires good intelligence gathering and communication between the police and probation service, as well as sufficient resources to continue monitoring of the offender. It is probably here that the system breaks down.
Whether or not you agree with how this works, it is for these reasons that it is referred to as a life sentence and really does mean life (if not always in prison).
Of course, there are some people on whom the Home Secretary set a 'whole life tariff' (e.g. The Moors Murders) as their crimes were considered so heinous that it would not be in the public interest that they ever be released. However, the European Court of Human Rights has deemed that the Home Secretatry does not have the power to do this and such tariffs are now open to legal challenge.
Remember also that in the UK the mandatory sentence for murder is 'life imprisonment'; there being no distinction between the 'type' of murder committed as there is in some other countries. Hence, the tariff allows for some sort of 'distinction' in the circumstances, whilst ensuring all people who are convicted will be subject to a life licence.