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Jaguar don't list relay locations in the XK Driver's Handbook as they previously did for earlier models. According to the Electrical Guide, the starter relay is in the Power Distribution Box (see item C):
A bit more digging, and I found this pic of a 2007 PDB:
It's hard to read the R1, R2, etc., stamped beside the sockets, but I found this drawing of an X-Type PDB which looks very similar (probably sharing components):
That implies that R10 is inside the PDB, i.e. it isn't socketed. Which probably means you have to swap the whole PDB to change it.