//When a judge hands down a sentence for, say, 12 months, he/she knows full well that only half of it will be served.//
Actually, ken (and everybody else who probably doesn't realise) he/she knows full well that in a large number of cases only one quarter of it will be served. Those sentenced to at least twelve weeks but under four years can be released, under a tagged curfew, having served one quarter of their sentence (subject to having served a minimum of 28 days). The maximum remission under this scheme is 135 days. This means anybody sentenced to 540 days (roughly 18 months) or less will serve a quarter of their sentence in prison, a quarter of it on HDC and the remaining half "on licence" (during which time they are supposedly supervised by the probation service, but otherwise free of restrictions). Those sentenced to more than 540 days but less than four years will be released on HDC 135 days before their half way point.
There are some exceptions to the right to HDC but the vast majority of those sentenced will be eligible for it. The government's waffle paper about it is here:
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/790670/home-detention-curfew-pf.pdf
A slightly easier to read version is here:
http://www.prisonreformtrust.org.uk/ForPrisonersFamilies/PrisonerInformationPages/HomeDetentionCurfewHDC
As an aside, when it was first introduced, HDC was only available to those sentenced to four months or more. In true civil service style, the designers of the scheme did not do a few simple sums. Thus someone sentenced to twelve weeks would only be released after serving six (the half way stage). Whereas someone sentenced to sixteen weeks would be released after four weeks (half off automatically and another quarter off under HDC). There followed the ridiculous farce of advocates pleading for their clients to be subject to a longer prison sentence as they would actually be released earlier!