Is the DVB-C tuner in a TV or a computer?
If you pay attention to the channel allocations on DBR you will see that the band of cleared frequencies, some of which are used for extra DOCSIS capacity (C037-C045) are not adjacent to the existing DOCSIS range (within the C004-C028 range). DBR uses Manchester data, so is a former Cable & Wireless network. I believe similar patterns exist across all former Cable and Wireless regions. As far as I know, some former Telewest regions use entirely different frequency ranges for DOCSIS, although most of the former Telewest regions do use a similar range for DOCSIS with the cleared capacity being adjacent to this range. There is no advantage to having such a wide band of adjacent frequencies. It is far too wide a range for all the channels to be bonded.
Fair enough for a guess but that isn't what is happening. A quick check of DBR channels would have proved that.
I would expect to see a lot of red. The Vxxx names are assigned randomly. The clear names will have been in use at the time of the most recent scan. The red names will have been in use on previous scans. Often fairly recent scans. The amount of red would only show a trend if the Vxxx names were static but they aren't.
That would be a very time consuming exercise. I've heard of Easter egg hunts but an Easter wild goose chase is a new one.
I believe that C001 is used for the first available channel above the FM band plus or minus the frequency offset used by Virgin in the local region. So your starting point is correct. The upper frequency differs between regions, with some going as high as 843Mhz. Maybe you should take a cautious approach and set the upper limit at 860MHz. That's only 94 frequencies for your Easter goose chasers to track.
Getting back on track, my point was that the trend you thought you saw might not be correct.
The current MPEG4 conversions of SD channels makes sense as it is a more efficient use of the spectrum, maybe that's all it is.
Now the reason I asked if your DVB-C tuner is in a computer. If it is, you can use it to extract the frequency table from the NIT for multiple Virgin networks. It would probably only take a minute to complete the scan. But, if you prefer the labourious Easter goose chase, go for it!
So, coming soon, more efficient usage of the spectrum.