There isn't a general legal requirement for your employer to give time off for medical appointments. Whatever entitlement you have to time to visit the doctor will be governed by your individual employment contract.
You should check your contract and staff handbook, as this may specify what the regulations are in your workplace. Some employers may require you to take annual leave, or take the time unpaid. On the other hand, many employers will be flexible on this, and it may be worth asking your employer if you can make up the time later.
These require employers to make 'reasonable adjustments' to prevent disadvantage to disabled workers. If your employer is not supportive, ask for advice from your union, a Citizens Advice Bureau or a Disability Employment Adviser at your local job centre. Reasonable adjustments could include greater flexibility in your hours to cover time off, or availability of time off (paid or unpaid) when needed. See our section on Disability Rights.