Really depends on what you have been maintaining the rest of your levels at recently. If it was me type 1, pump and reading at 21 I would be really, really worried and would be doing everything in my power to get levels lower, because my normal levels rub be=tween 5 and 8.
If you are running constantly at these levels it may not be such a dire emergency, but a need for a check up with the person that monitors your diabetes next week.
What have your levels been like for the past week, is this a dire shock to you? Have you and do you regularly check your feet, as all diabetics should do?
Might be worthwhile phoning NHS direct, but honestly there a lot of factors to consider before going to A&E.
I expect others may think differently, but I know that on a Friday night or Weekend at A&E that you would not get priority treatment, as such...especially if you have not had brilliant control on an ongoing basis prior to going there....
Harsh, and only my opinion, and not a medic...