Alan,
As you have seen from the PilotAware website, the external antennas are normally available as a pair, without the cables and accessories, but in light of the current unprecedented demand and the short notice given to suppliers by the CAA, the existing stock of these antennas has been packaged as ‘Rosetta+ Kits’ - which comply with the requirement for the rebate claim to be against the purchase of ‘A Singe EC Unit’, - thus maximising the potential grant relief for PilotAware customers.
You are correct, in order to keep the price of the Rosetta+ packages down, the screw-on end-fed antennas normally supplied with Rosetta have not been included in the packages, but they are available separately.
If you can’t wait for further stocks of external antennas (without the extras) to come back into stock, the 1090MHz one can be replaced by a standard transponder antenna. The only viable alternative for the 869.5MHz P3i side would be one of the external antennas sold by various companies for use with FLARM transceivers. Although these will be tuned to 868 MHz, they should work fine on the slightly higher P3i frequency.
Hope this helps.
Regards
Peter