At the moment, the words are used interchangeably, as there is no legal distinction between a Counsellor and a Therapist. Many people typically shy away from the word 'Psychotherapist' as it semantically has a closer connection to professionals such as psychiatrists and psychologists.
Many Therapists recognise this and call themselves Counsellors - a term which people typically associate with a more informal means of therapy; of 'talking to someone'.
However, as people in the UK become more aware of the benefits of Therapy, the stigma is slowly dissipating, much to the delight of those who have often spent 3 - 4 years studying and practising in the profession.
It is likely that the distinction between the two names will become clear very soon, however. Current legislative moves towards the standardisation of psychotherapy training will mean that a Psychotherapist has a set level of training which will typically be higher than someone who may call themselves a Counsellor.