Well keef,
If the ministry think the reason Mode S is required is so TCAS will work, and they are making little planes put it in for that reason, the only logical conclusion is that they are going to force little planes to install TCAS too. Otherwise as is well known, Mode C will work perfectly well.
Thank God logical conclusions mean nowt to government departments, or in this case you can only hope so.
Doesn't the CAA say little planes shouldn't use Mode C in airport environments because it upsets people with TCAS???