The term bandwidth can be used for several different things, so to explain it in easy to understand but not quite accurate
terms for those who don't know anything about it.
In this instance there are two types of bandwidth;
The first is the frequency bandwidth also called the frequency or RF spectrum this is restricted to a set number of frequencies by physics, in a network such as ntls and Telewests this is termed a Carrier as it carries the TV signal or the data for the modems etc. and is measured in Hertz, MHz KHz etc
The second bandwidth is the data bandwidth which is about the amount of data that can be carried within each carrier measured in the good old bits and bytes Mbs MBs etc
The restriction in the HFC networks isn't on the physical coax but the frequency range of the amplifiers themselves many of which only go up to 750MHz, this means that the majority of the frequencies available are already being used by the cablecos, as well as stuff already in the home such as VCRs, DVDs, STBs etc.
To take Ianathuths suggestions one at time;
tbh I don't know enough about it to comment.
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Duplicating fibre all the way to the nodes to provide double the capacity for TV and broadband.
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again the restriction isn't the capacity of the fibres to handle the traffic but the electronics that feed into the lasers, if you had two fibres going into a single node you would still only be able to use the same frequencies once unless you doubled up the optical receivers amps etc and redesigned/rebuilt the network, which would be extremely expensive.
No real benefit gained on this one, we've already said that bandwidth isn't the issue on the fibre, the continuing work to resegment is increasingly resulting on a single 500 home area having its own uBR in the hubsite moving it into a street cabinet makes an extremely expensive bit of kit vulnerable to the vandalism etc that all the cabs are subjected to.
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Continuing fibre from nodes to cabs.
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This is currently happening, a lot of the 500 home areas are being broken down to smaller groups, this is being driven by BB but will in the future hopefully be driven by VOD.
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Having STBs that fetch TV from servers at the Hub (BOD) similar to VOD so that not all TV channels are sent to all customers, freeing up bandwidth for other purposes.
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Very good idea, one that I'm sure is being investigated possibly in line with Altis' suggestion about CAT5e but again the existing coax is a lot more robust for the ducting and gardens and iirc is capable of a better response over longer distances than CAT5
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In a way it is a pity that there are competing platforms providing services. Just think of using satellite for most of the download for TV and broadband and cable or DSL for upload and return path.
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Again a good idea but I'm not convinced that the monopoly and mergers commission would agree
