If your looking to upgrade for a cheap price, take a look @ Mustang Cobra rotors on e-bay US. You can put on 13" Brembo rotors for about $200US or so & then you need a spacer kit which you could pick up from any local shop. But you need 17" rims or higher to fit these rotors. If you looking for just a stock up grade checkout these, I have them on my Z & they work great. This by far the cheapest way to get a big break setup for the Z that I've seen thus far, & with the Euro US conversion this would be even cheaper for the UK.
http://cgi.ebay.com/ebaymotors/NISSAN-300ZX-89-96-Brake-ROTORS-PADS-SS-LINES-PKG_W0QQcmdZViewItemQQhashZitem35a8551655QQitemZ230457415253QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
Here's the Mustang Cobra Link:
http://cgi.ebay.com/ebaymotors/Mustang-Cobra-SVT-03-04-Drilled-Brake-Rotors-and-Pads-F_W0QQcmdZViewItemQQhashZitem35a87ace6bQQitemZ230459887211QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
These are for actual Brembo slotted rotors for the front:
http://cgi.ebay.com/ebaymotors/Brembo-Front-Cross-Drilled-Rotors-94-04-Mustang-Cobra_W0QQcmdZViewItemQQhashZitem53e052d05cQQitemZ360245809244QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories