Isp is one the LK-selectable Brand Names contained in the CPC BIOS.
Isp might refer to ISP KG Dieter Lather, which operates (from 1975 until today, 2010) as european distributor for ORION products. The interview doesn't confirm whether or not Amstrad was referring to ISP KG. However, there is no other big electronics company known to use "ISP" or "Isp" as brand name and being active back in 1983.
Considering that Amstrad did have some relations with ORION, it doesn't seem too unlikely that they also planned to work with ISP KG. Although, as far as known, ISP never rebadged the ORION products as ISP products, so there would have been no real requirement for the Isp boot message.
Some other solutions may be found on wikipedia (although currently, 2010, wikipedia doesn't give a better solution),