Hi and welcome!
I cant answer specifically about diabetes. but if you are planning on moving here permanently then you will need to make some provision for health care eventually.
Ive copied some information for you below that gives quite a bit of information relating to health care here.
About Medical Treatment in Spain
The Spanish health-care system is very good. Most hospitals are modern and well-equipped and the doctors and peadetricians are excellent. There are a few differences in policy between the British and Spanish health systems, but overall they are very similar. The biggest difference is the level of nursing care available in Spain. While Spanish nurses are well-trained and efficient, they simply do not perform many of the duties carried out by British nurses, and many tasks (particularly personal care and feeding) are carried out by the patient's family instead. All hospitals allow one companion to be with the patient 24 hours a day. Visiting times vary for other visitors.
If you are planning to live in Spain then do remember that many medical staff do not speak English, even in resort areas. In some tourist towns a voluntary translation service is available for doctor's appointments, but that will not be available at all times. It is a good idea to find out whether or not such a service exists in your local medical centre when you first arrive in Spain, and if so, at what times it operates.
The emergency phone number in Spain is 112
Entitlement to free Healthcare
Before moving to Spain, you should make sure you apply for the new European Health Insurance card (EHIC), which replaces the old E111. Application forms are available either at the Post Office, or at the UK Department of Health website.
A EHIC will cover you for emergency healthcare treatment in Spain. It is really for tourists, but will ensure that you do not end up having to pay for treatment during your first few weeks or months in Spain. It will not cover you for most types of non-urgent treatment such as treatment for ongoing conditions, full maternity care or vaccinations and you should not rely on it if you are living in Spain.
If you are planning to retire to Spain, if you are unemployed before you leave the UK, if you are a student, or if you are self-employed and planning to work in Spain, then you will be entitled to 2 years worth of full healthcare. See The Department for Work and Pensions for details. NB With the forms you get from the DWP you'll still need to get registered at the offices of the Spanish seguridad social. This process can take some time, so be sure to get your EHIC card anyway.
Private health insurance is widely available and on the coasts at least there are many schemes specifically aimed at British ex-pats. Do read your policy carefully though as you may find some things you will need are excluded. Some policies only cover you for emergency treatment in the home and/or an ambulance to the local hospital for example and you may find that you have to pay for the hospital treatment after you have received it.
Most ambulatorios or centros de salud (health centres) have a dentist who will provide free treatment to people registered with the social security system. You may also find dentists offering emergency treatment in Spanish hospitals. We have received mixed reports about these dentists - some have complained about long waiting times and "brutal" treatment, others have praised the work. If you have any problems getting an appointment with the dentist at your local health centre, then private denatl treatment is widely available and prices are low compared to private treatment in the UK. For private dental work, expect to pay around 10 euros for a checkup, and 40-50 euros for a white filling.
Many medicines that are prescription only in the UK are available to over the counter in chemist's shops in Spain, including Asthma inhalers and antibiotics. If you are registered for free healthcare under Spanish social security, you can get medicines you need on prescription for a 60% discount, or for nothing if you are a pensioner.