A lot of moving parts
Part of the issue is that there is no single provider for NHS Systems, a lot of the time someone will do 70% of what is required. Cerner may or may not include a Pharmacy component which means that a separate Pharmacy system would have to be sourced & would (hopefully) be able to interface with Cerner. Another central system might have pretty much everything but some of it not as good as a specialised system so that part doesn't get implemented.
Back in the day of on-prem system it was possible to use an integration engine to extract records from one system, do a bit of formatting and then pump that into a second system, rinse and repeat until you have several systems that can pull the relevant information from other systems. These days I would guess a lot is cloud based and a lot of what could have been done by internal IT is now a paid for service from the supplier so integration may not be as simple.
With the under-staffing in the NHS front line at the moment, people may also not be able to get trained on systems so what could be a 2 minute process ends up being 10 minutes & a call to IT.