Regrettably, according to IMDB, the series never seemed to make it to UK DVD. It was scheduled to be released on 26/09/05 but then delayed until the following month and then still not released. There seem to be no web references to a UK release yet, strangely, it was available in Oz in 2007 (on a Region 4 DVD) and it remains available from the ABC website. So, if you've got a multi-region DVD player you might want to consider ordering from Oz:
http://shop.abc.net.a....asp?productid=747760
(ABC will ship to the UK. See the relevant page of that website for carriage costs).
Wikipedia confirms that only the Region 4 DVD is available:
http://en.wikipedia.o...lp_%28UK_TV_series%29
Given Chris Langham's 'fall from grace', I'd regard it as unlikely that the BBC would now consider issuing 'Help' on DVD within the UK.
Chris