Like the ones in the link below. Wondering if there is a UK supplier for these?
I know they are initially expensive, but they are a bigger size. Maybe somewhere does them in the same size but of the two piece/floating variety?
Goodwin Racing BIG BRAKE Conversion for BREMBO for MX5-ND 2016-2023
The only place you can get them is from Racing Beat Europe who will import them from Good-Win Racing:
One thing that is off-putting with those brake discs is the unswept area. It’s not completely covered by the brake pads so you will see rust eventually forming on the inner area:
https://www.124spider.org/threads/brembo-caliper-owners-ready-to-go-big.23010/page-2
Fiat/Abarth 124s share the same brakes as the ND 
You can get standard size two-piece discs directly from MiataSpeed:
They also have the same unswept area issue. The only two-piece discs I’ve come across that potentially may not are Project Mu’s SCR-Pro disc since they are coated with rust-resistant agent. If it helps, Project Mu also supply AutoExe’s own Sport Brake offerings:
@ckleanth: FreakyParts do not offer two-piece discs for the factory fitted ND Brembo calipers.
I didn’t know if they did or not (because I own an NC and don’t know enough for the ND with brembos) but I figured if someone would know it would be them.
Strange that the MiataSpeed rotors are 280mm, so the same as factory? And they still have an unswept area
They are the same diameter as factory fitted 2.0L brakes. The unswept area is next to the hub and the following link shows photos of what they look like after being used for awhile:
The front disc photo shows up in the first post and the rears just after scrolling down halfway.
Project Mu’s are the same size as factory too. I was close to buying a set of front and rears then changed my mind after discovering the weight difference was negligible. If it helps, the MiataSpeed are lighter.