Is this available on DVD?
Yes, it's available on Anchor Bay's 1999 DVD release Tenebre and their 2001 release Tenbre / Deep Red. Both contain the 101-minute uncut version.Note that this film (or at least the edited, English-dubbed version called Unsane) seems to be in the public domain, which means any distributor can legally sell copies without paying royalties. Beware. Some small distributors market copies of public domain films with poor picture and sound. Others are more reputable and deliver good transfers of the best available prints. Shop around.
What are the differences between the old British BBFC 18 Version and the Uncensored Version?
Dario Argento's "Tenebre" was released by several labels in a slightly shortened version on VHS and DVD as well. Two violent scenes needed to be removed while three more shots/scenes were also removed but these scenes are missing in other countries as well. The reason is the bad quality of the available master. The old tape by Videomedia only lacks one violent scene (arm amputation). Subsequently the tape was listed as a video nasty. The uncut version has been released by Anchor Bay and Arrow in the UK, on both DVD and Blu-ray. A detailed comparison between the old VHS by Noveaux Pictures and the uncensored version with pictures can be found here.
A comparison between the pre-cert VHS by Videomedia and the Uncensored Version can be found here.
Does an alternate cut exist?
The very rare Israeli tape contains several scenes with, compared to many other releases, different footage. Whether this version has also been released in other countries couldn't be determined. No clues that point into that direction could be found. Book texts and letters, written in Italian in the familiar versions, have been replaced by English texts on the Israeli VHS. Among other scenes, the scene with Maria discovering the killer's archive in the basement contains alternate footage. Furthermore, the VHS lacks scene parts, probable due to jumpcuts and wear and tear. A detailed comparison between both versions with pictures can be found here.