What about if the cabinet is sitting directly on the floor so that you can drag the extinguisher out horizontally without taking any of the weight and then lifting it when it is clear of the cabinet? I haven't tried it, just speculating on a solution. It's an interesting issue but I haven't seen many extinguisher cabinets over the years so I wouldn't have guessed it was a common problem or, indeed, one with published UK guidance - though it will be interesting to see if any comes to light here.
I do have a problem with them being mounted higher up walls than recommended, generally (i.e. when not in cabinets). I was in my kids' primary school hall one day standing at the back of the weekly assembly to which parents were invited when, lo and behold, one of the parents who was crowding in a little along from me knocked a 9 litre water extinguisher off the wall. The extinguisher was mounted with its handle at about shoulder height (adult shoulder height - 1.5m) and it glanced off a child's shoulder before crashing to the floor. Luckily it was a slight glance and no one was hurt but the potential for harm was immense and it was very fortunate that the potential was not realised. Off course, the bracket was loose in the wall, as is so often the case, and it was not the parent's fault as they only brushed against it to make it fall.
Also, when mounted at this height there is the potential for things to be put beneath them and whether permanent or temporary, such items may require a person to adopt completely the wrong stance when taking such a heavy item from the wall, having to lean over the item to reach the extinguisher.
I recognise the comments in favour of the higher positioning but, for me, the best solution is to keep them low (and out of cabinets! - how are you going to wedge a fire door open with the thing if it's in cabinet?)
Stu