Programmes that are signed almost always tend to be repeats, shunted to the early hours, so that deaf people have to record and watch them later. I don't think that is pandering to them, or forcing it on the rest of the population, frankly - especially given the limited number of programmes broadcast in this way.
It's not a question of not being able to read subtitles - as has been mentioned by others, signing is much more natural than reading for many deaf people. And it's also worth bearing in mind that, effectively, BSL is the first language for many, not English. It's their natural means of communication, so why shouldn't they get a few hours of programmes?
Welsh and gaelic speakers, after all, get far more provision, to the extent of separate channels. Signing is obviously not cheap to do, of course, so we're a long way off being able to provide a whole channel with everything signed, which I'm sure many would appreciate. In the meantime, it seems a little mean-spirited to begrudge people the few signed programmes that there are.
As far as technological options go, I don't think the current interactive standards presently allow for one video to overlay another; you'd need a whole extra stream with multiple videos, like the news multiscreen, and MHEG 5 1.06 to do the zooming, but you still couldn't overlay, you'd be taking up a stream (and I bet there are plenty who'd moan about deaf people getting that....), and it would be a bit of a kludge, available only to people with appropriate equipment.
Broadcasting as it is ensures that nothing extra is needed for people to access signed programmes.
There has been work done on avatar-based signing, where it can all be done automatically, with the signer being generated by a set top box, but that's a long way off as far as I gather - and of course requires quite a lot of power in the receiver if it's to generate a convincing animation that can convey the information a real person would.