Well most people would say that removing the centre would increase the flow, but to be honest, given the opening lift and the viscosity of coolant, it won't actually make any difference. The only way to increase the flow by playing with the thermostat would be increase the size of the opening. ie remove the thermostat altogether (but ensure the seal seats properly.

This will of course marginally increase warm up time as the low volume 3 stage cycle is defeated.

But I don't think the pinch point for coolant flow is the thermostat. Don't forget that although only the block, head & pump remain essentially the same on the WRC engine, Mountune/Cosworth spent a lot of effort reworking the standard (not FRS, was it homologated?) waterpump to increase flow, including increasing the size of the waterway into the block.

No for your trip, given the timescale, I'd be down a breakers finding a Mondeo/Escort thermostat housing and getting a couple of sets of seals. Unless you can detour to Guildford on the way to the ferry to borrow one of mine.

Martin