I would have no hesitation about wearing tweed at Sandown. It might be inside the M25 but its not as if you're turning up to main building in wellies and a flat cap is it? Its a field. On a race course. As you said there are few enough occasions to wheel out the old tweed and I think Sandown is one of them.
As a back up just make sure you carrying a healthy supply of dam-yer-eyes. If anyone dares to raise an eyebrow just be confident in the knowledge that you are above mere social norms and that the very fact that you are doing it has caused it to become acceptable.
Far too busy yet again, inspection after inspection. Disappointed, was really looking forward to seeing old drunk faces in the bars escecially the RSA, 1st Regiment and 19 Regiment tents. Hopefully our gang will make up for it at the Gunner bar at this years Army v Navy.