Huge efforts have been made to link the systems in the different countries together but the set ups are different wherever you live.
England, Wales and the Isle of Man use the NHS system, however, depending on which health authority you're in, what level of access you've been granted and what country you're in, you can see some or none of the patient's record.
A hospital might be able to see a patient's GP Practice in order to trace the record when the GP is next open, an address history, NOK details (only if the patient gave them and if the GP requested them when the patient registered) and contact details.
Scotland, Northern Ireland and England & Wales use slightly different numbers.
Someone in England can't access the records of a patient from Scotland and the same with Northern Ireland.
What we, in England would call the NHS number (10 digit code starting with a 4, 6 or 7), the Scottish version is Community Health Index Number which is generated around the patient's DOB and Northern Ireland use a Health & Care Number starting with a 3.
The systems generated around and with those numbers were different and huge sums of money have been spent trying to link them together.
Different hospitals have different computer systems, using different operating systems, programs and with different ways of recording information depending on what packages they bought or developed.
A system called Lorenzo which can link different programs together is being used in some hospitals but it's centrally controlled, prone to shut downs and if someone makes an error with a patient's record, then the error goes across the GP and national database within minutes.
It's a bitch to correct.
It is also trying to replace a number of specialist programs and can't mimic the same functionality that the separate programs could.
The increased processing power required would make trust wide, prohibitively expensive upgrade a necessity - the costs to be borne by individual trusts and not central government.
It's a tangled mess.