Surely you would just move the up slow out and build an island in the gap, leaving the other three lines untouched. If you later wanted to build a fast line island, you would move the down fast out as required and build the new island in the gap. No need to demolish anything?The Midland mainline runs 4 tracks without gaps in between. If 2 platforms are built you can just shove the slow lines to the side. But you cannot do that if it's 4 platform. If it's 4 platform the fast lines need realigning. So this means if they go ahead without park go ahead with 2 platforms they will have demolish it and start over again. Apparently if Wixams don't spend the money they have been given they will lose it. That's what I read don't shoot me if I am wrong. Also if station build goes ahead with contingency in place you might as well do the 4 platform from start as you're doing almost all the work.
Alternatively, if you built a central island facing the up fast and down slow and a side platform for the up slow (which would probably give a better alignment for the fasts as they could stay exactly where they are), you could later add another side platform for the down fast without demolishing anything. If you really wanted to penny pinch you could even leave the fast side of the island as just a gap (similar to what has recently been built at Dore) and build it out when you add the other side platform.