Hi, if I remember correctly, dangerously high that you would need to go to hospital for is over 20mmol. You could give NHS Direct a call and ask them.
The "H1" will flash if your BG goes over a certain number that your meter has been set at to warn you that you have gone above this level. If you check your meters settings, you'll find a setting for "L1". There might also be an "H2" and an "L2" setting as well. The two "Hi" settings might be set at 8 and 9, these are to warn you that you are above the normal range. The "L1" and "L2" settings might be something like 3.9 and 2.??. First is to show you that you are hypo and the second that you are dangerously low. You can change these setting to the levels you want but probably best to leave them at default settings
If you have an Accu-Chec meter anything above 33.3 mmol will show as "E1". There is another for being dangerously too low but I can't remember what that warning sign is, you can look them up in the booklet you got with your meter. If you get these dangerously high or low warnings you need to go to A&E immediately.