I think the option not to be able to receive BBC channels should be available and then no TV licence. Surely with modern technology that's not beyond the wit of man?
For the programs the BBC make that I watch I'd happily give their channels up if it meant I no longer had to pay the licence fee.
The thing is that:
1: The TV licence isn't a BBC licence - don't for a second assume it would go away if the BBC ended up going subscription.
For one thing parts of it are already being hived off/planned to be hived off for other things, including government foreign policy*, and broadband, the government even raided a fund that was collected as part of the TVL but specifically for one purpose, and when it turned out there was an excess the BBC wanted it refunded to TVL payers (as it hadn't been needed for the purpose under which it was collected), but the government decided that despite it having been added for a named purpose, they would use it for another.
2: Making the BBC subscription/opt out totally blows the whole idea of a non commercial PSB out of the water. Why would a commercial/subscription model BBC bother with expensive documentaries and drama when it is far more profitable to go the ITV/C5 routes.
3: It would totally kill off any form of halfway decent (IE non music jukebox) radio for much of the country - most of the commercial broadcasters for radio wouldn't touch the sort of speech programming that the BBC stations have as part of their mainstay (IE unbiased money advice, health advice, gardening, let alone radio drama etc).
bsoltan: The BBC don't have as much to do with the infrastructure now, but IIRC guarantee the likes of the smaller transmitters/relays (whilst the commercial broadcasters can decide keeping a mux functional on a relay/small transmitter is not worth the cost, the BBC have to keep paying for one for their channels). They also still do a lot of fairly basic R&D and are one of the preferred testing partners for new tech because they're willing (and not only able, but IIRC required by their charter) to work on improving broadcasting - I vaguely remember at one point they were willing to share the Iplayer tech but were stopped in the name of "competition" (it's happened with various projects that would have been, or were before being shut down a great advantage for the viewer).
swiftmetal:
There's no need for test cases. IIRC pretty much every argument against the TVL because you only watch non BBC broadcasters has been tried and failed - the law specifically states Live TV not the BBC for a reason.
*BBC:World Service used to be paid largely for by a grant from the Government because it was seen as a good thing for diplomatic, economic and generally keeping Brits abroad (in the services, working for the government abroad etc) in the know. The government recently decided it should be funded by the UK TVL.