Caveman, all great questions!
1. As a general rule, I do not ever have sex without a condom.
First off, there is the pregnancy issue. She may be on birth control, however, and that is another issue altogether, so I will get back to the STDs. The problem with STDs is that many of them do not show signs or symptoms for a while before they become apparent, while others can show signs at times and then not be visually evident for a while only to resurface later. The ONLY way to be fairly certain she does not have an STD is if she gets screened for them and then SHOWS you the report saying she is NEGATIVE for any sexually transmitted infections. Just because she has not had sex in the past few months does not mean she does not have an STD. Even if it is unlikely that she does, there is still that chance...
2. As for the AIDS. So basically, AIDS occurs as a result of the HIV viral infection that progressed to AIDS, which is a worsening of the immune system and acquisition of other infections. HIV is the virus which you are asking about and it is a scary one! As for prevalence in England, it is very low. Sadly, however, transmission rates have increased slightly in developed countries recently. Still, prevalence is low, but it is still there!
You really would not be able to tell if she had HIV or AIDS by looking at her, especially if it is in the earlier stages. As the infection progressed, she would have some symptoms and would start to get infections that healthy people don't tend to get. Still, considering that many people with HIV do not even know they are infected, you certainly could not tell for certain without testing.
3. HPV. The prevalence of HPV is hard to actually estimate. There are MANY strains of HPV, some of which tend to be worse than others. The 70% number you used, I believe, is of people infected with HPV at some point in their lives. The thing is, HPV usually clears on its own. When it doesn't, however, is when people run into problems. We do not currently know why it clears in some, but not others, but that is how it seems to happen.
For both HIV and HPV, sexual transmission is highest in anal sex and lowest for oral sex. Vaginal sex is somewhere between the 2.
I am sorry for the length of this post, I just wanted to answer your questions as best I could! As for being scared to have sex, I will say this. Even knowing all of the dangers of STDs, I still have sex and love it! I'm just careful who I do it with. I also am sure to use a condom everytime without fail. If I don't have one, I don't have sex. Period. Hope this was helpful and if you want more clarification on an answer or have any more questions, feel free to ask!