There is no general legal requirement for your employer to give you time off for medical appointments. Whatever entitlement you have to time off to visit the doctor will depend on your individual employment contract.
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.
If you have a disability which requires you to make regular medical visits, your employer has a legal obligation to make 'reasonable adjustments' to prevent your disability putting you at a disadvantage in the workplace, when compared with your non-disabled colleagues. One reasonable adjustment could be to allow greater flexibility in your hours, including time off to see the doctor, paid or unpaid. What is 'reasonable' will depend on your particular circumstances and those of your employer.
If you are in a union, speak to your rep who will be able to support you in making your request.
If you are not in a union, you may be able to get help from a disability support worker, or one of the many specialist charities. Also have a look at Disability Rights UK's Access to Work webpage to see what financial support might be on offer for your employer to help accommodate your disability.
Some employers – those who have signed up to the government's Disability Confident scheme – ought to be more flexible in meeting your needs as a disabled worker.
Unions are expert at supporting disabled workers. Browse our Union Finder tool for some of the benefits of joining a union and to decide which union best suits your needs.