NB the source of information for the above was in most cases the YHA's own web pages, buried in the booking forms.
See for example YHA Alfriston. On the main page for the hostel:
http://www.yha.org.uk/find-accommodation/south-east-england/hostels/alfriston/in...it only tells you the 0870 number. But click where it says "BOOK NOW", then on the next page select individual bookings and proceed to the next page, it then gives the geographical number (for calling from outside the UK).
Any remaining gaps were filled from:
http://links.open.ac.uk/yha/list.htmland in a small number of cases by searching the web for the name of the hostel with the known STD code for the area.