The 14" wheels you have are a great choice for the road, and will be good on track too. 13", without adjustable suspension would lower your ride height by around 10mm, while the 15" wheels need shallower profile tyres which lack the compliance that helps the seven ride so well on the 'average' British B road. Stick with the 14s for now, and get a set of Toyo R888Rs. You can add a second set of wheels and tyres later if you want to try some alternatives.