Only true until the end of the center section and back box.
Just got a 98 DC2 center section from honda and the second silencer in the center section is actualy only 2" all the way through and the collector flange size is the 2.25" donut.
Goes the same for the EK9 back box witch has a flange of 2.25"
Only difference between JDM DC2 and EK9 centers is the lenght as the DC2 Center sections is deffinatly 2.25" / 57.2mm external diameter.
Id say the UK DC2 centers are probably the same as a civic Vti or same dia as the JDM jus not as well made, as the JDM centers are actualy quite well made i.e without big crimps where it bends.