I've talked to Kevin at EB Spares a while back. They don't have V6 headers for the 155 V6. They MAY have some stuff for the 156, but I doubt it would be headers for the 156 V6, probably just the mufflers.
I was thinking of modifying 164 headers, but the 164 engine tilts at a different angle.
One other solution is to send the standard manifolds to extrude hone and have them shave off more material. But its about US$500, your manifold and about 4 weeks! Too much of money and time.
Yeah we drive on the same size as the Brits, we were a British colony.