I find it had to believe that the burden of proof in rape cases is that different between the US and the UK, which leads me to suspect that the difference in conviction rates might be down to US prosecutors being less likely than UK prosecuters to proceed with cases that are diffcult to prove (e.g. when it's one person's word against another). Given the statistics, it looks like the British prosecutors must be bringing a lot of cases that they know they have little realistic chance of securing a conviction on.
If that's the case, then those prosecutors are engaging in a criminal level of negligence and professional misconduct in that they are completely failing in their duties to their clients and to the cause of Justice.