UK Pork roast: why can't I make crisp crackling?
Whilst I do not know about the practices in Denmark that would make the achievement of crispy crackling there, I can comment upon the Chinese and UK angle, and, offer some tips for you to get good crackling with pork in the UK.
There are a couple of characteristic differences with Chinese practice. Their pigs are not as genetically refined for low fat meat as the UK and European ones. The Chinese place great importance to a thicker layer of fat under the skin of their animals and this is a factor that helps make good crackling.
Secondly, they have a much larger proportion of meat sold &quot;loose&quot; rather than plastic wrapped in some remote factory and then distributed through a refrigerate distribution chain. This means that the skin of the pigs is rather drier too than a normal plastic wrapped piece would be. Skin moisture content is a critical factor in achieving good crackling. The Chinese promote belly and shoulder over leg joints as they (rightly) consider them better for flavour and cooking.
To get good crackling using typical UK supermarket plastic-wrapped pork -
Try Shoulder rather than leg - It's usually got more fat  underneath the skin.
A slab of belly pork is even  better although pretty uncommon in UK supermarkets nowadays. Those with butchers onsite (E.g. Morrisons) will sometimes have whole bellies to cut you a piece that you like. If you have a Chinese supermarket in reach of you, they generally have very good butchery counters and you will generally be able to get much fattier (tastier) meat from them.
Look for a good layer of fat under the skin.
Unwrap the meat the day before roasting, scald the skin with lots of boiling water by pouring it over it (not immersing the whole joint), dry it and then score the skin with a very sharp knife about 2/3 the way through the underlying fat. Rub generously with salt (You will wipe off a lot of this before cooking) and put uncovered in the bottom of your fridge skin side up.
The fridge will suck moisture from the skin. 12 to 24 hours is good. Wipe off salt, use a little neutral oil on the skin, season with white pepper and a little sprinkle of mustard powder, rub into the cuts. Crank up the oven to max and start the roast off at this for 20 mins (Open windows if you have it hot enough it'll be smeeching).
Turn down to circa 170/180 deg C for remainder. Use lard for the roasting fat if their isn't lots of fat rendering from the meat. Baste the meat often. Also, remove non-fat liquid from the roasting tin as you go. Using a clear heatproof glass container enables you to see the fat liquid split and to recycle the fat off the top back into the roasting pan.
When meat is resting do  not tightly cover or you will steam the crackling making it soft. If the crackling isn't crisp enough when removed to rest, slice off the crackling in one piece and put it under the grill for 10 minutes on high - watch it though as you don't want to burn the edges too much.