This is something that hits very close to home, as I'm in the same path as Pieter and your father and I see that path getting me to the same end. If that happens, I'd like to end on my terms, not as the husk that you describe and not after having my family go through all the pain that you describe.
It's intended to make it so that the relatives of someone who travelled abroad for assisted dying are less likely to be prosecuted under the Suicide Act 1961.
See http://www.theguardian.com/society/2010/feb/25/assisted-suic...
Pretty much all attempts at legal change (whether through legislation or through court action) in the UK have been a failure. It pops up every few years in Parliament and is then roundly voted down.
The Terry Pratchett documentary was challenging to watch, but very well made. At least it gave rise to a somewhat more rational discussion of the topic than they often are. It's sure to be out there on a torrent or YT for anyone who's not seen it.