UK Passports issued in the UK now have 'IPS' (Identity and Passport Service) as the 'Issued by' location. Previous locations were 'UKPA' and prior to that, the physical location at which it was issued.
UK Passports issued outside of the UK, have 'FCO' whereas they used to state the location.
Whereas passports issued in the UK have always included the most advanced security features, for various reasons (cost mainly), the documents produced abroad have not. There is also the view that in some countries, the facilities for producing a UK Passport are less secure than in the UK.
The US DHS in particular can be difficult with passports from developed countries produced by Embassies / HC outside of the developed world.