In addition to what others have said: many GP surgeries operate (in agreement with the other surgeries near them) what are called "catchment areas". There is often scope for negotiation but GPs aren't obliged to register anyone who asks to be registered. There are various reasons for refusal - the practice list might be full, for example. The home address might be too far from the surgery (this is for home visiting, which is not GP home visiting these days, but for access to any other local primary care services, such as wound dressing). The local Integrated Care Board (which replaced CCGs in 2022) will have information on which practices are accepting registrations from a given address.
Integrated care boards (ICBs) replaced clinical commissioning groups (CCGs) in the NHS in England from 1 July 2022.
www.nhs.uk
The other thing is that, once registered, when a referral is made to a specialiost service the patient should be given a choice of where to be referred. The practice will always have a locally commissioned service that's probably used by most/all of its patients, but that doesn't stop anyone from asking to be referred anywhere. The criterion generally is that the chosen referral site already has an NHS contract - which will be the case for diabetic services. This can however sometimes not be quite so easy in practice, because the cost of the secondary service is paid by the local ICB. If the cost is significantly higher they may baulk.
All NHS services are subject to regular inspection and you can find information about how they perform on the Care Quality Commission's website -
https://www.cqc.org.uk/