This is a very common question. The RCSLT’s advice is to contact your local speech and language therapy service as they may be able to arrange an observation session.
However, you may find this difficult to arrange – and the universities do recognise this issue. Course admissions tutors will look at your understanding of the profession and the needs of people with communications disorders, and also your ability to communicate with them, rather than whether you've simply observed therapists at work.
This means that any reading you have done, voluntary work and other relevant work experience are all regarded as relevant. Observation sessions are therefore not a necessity if you employ other methods.
Useful background experience can be gained with a variety of the client groups of speech and language therapy, e.g. children and adults with a learning disability, older people, and people recovering from a stroke or head injury etc.
Contact your local volunteer bureau or try to get some work experience in healthcare, a local nursing home or school. Local stroke groups also often need volunteers. The following organisations concerned with the needs of people with speech and language difficulties may also be able to provide information on opportunities for voluntary work:

Financial information for students

Read our guide to funding for current students across the United Kingdom and details of the changes for those commencing study in England from 2017.