Unfortunately, despite the apparently genuine original question, this subject arouses so much passion among pilot forum inhabitants that few people are going to post yet again and start yet another long thread...
But let me say just one thing
Also, it will be able to decode all sorts of things about you and your aircraft, so that NATS can start to levy navigation charges
the above is b0110cks. The radar coverage doesn't exist to enable transponder returns to be used for any kind of enroute billing.
Route based billing would in any case be a total nightmare to implement since flight plans are not mandatory for intra-UK VFR and that is just the beginning of what would have to be put in, at a cost which would be far too enormous to ever be recovered from UK GA activity. So let's not do this particular piece of b011ocks all over again... please!
The Elementary Mode S return is a 24-bit number which can be decoded into the tail number e.g. G-ABCD. Apart from the Mode C altitude info, that is all that ATC are going to get from your Mode S transponder.
IF we do, one day, get VFR enroute charges (as I believe they have in Canada) it's likely to be a flat quarterly/annual charge per aircraft, say £50 per qtr or whatever.