You have an absolute right under GDPR to see your medical records.No you don't.
GDPR devolves health record access to the pre-existing Access to Health Records Act, read it and you'll see that the latest amendment to take account of GDPR is dated 25.5.18, the day that GDPR came into force.
If you read the legislation you will find that Section 5 states that records can be withheld if releasing them would cause 'serious harm' to anyone (including a doctor or a third party, and not just the patient), and that the arbiters of what is harmful are the doctors (whose reason may or may not be legitimate). Neither the patient nor their solicitor are allowed to know this reason, so even if you exercise your right to take it to court, you have no means of arguing your case. Never mind that most patients won't have thousands to spend on legal fees each time they need to access their records.
This was also confirmed to me by a lawyer at Mishcon de Reya who specialises in medical records.
Finally, the NHS don't need to use any of this anyway, all they have to do is deny that the records exist, and then you can't complain that you haven't been given them. The same applies to correspondence, if they reply you have something to challenge, and evidence to show others, much better to just ignore it, then you have no proof they didn't answer.