I have found this relevant information:
Entry
The ambulance service in the UK is organised on a regional basis and run by district health authorities or NHS trusts. Each ambulance service sets its own entry requirements, and recruitment procedures will vary between regions. It is essential you contact your local ambulance service for further advice regarding their specific entry criteria.
Some ambulance services will only recruit technicians from existing employees already working as ambulance care assistants (see job profile); contact your local ambulance service for details.
If you passed your driving test after 1996, you may need to gain an extra driving qualification (known as C1 and D1) which will enable you to drive passenger carrying, medium sized vehicles. Some ambulance services may provide support to candidates through this extra driving qualification, however, this is not standard practice. All new entrants must pass a medical, and many services conduct a police check.