Tilly,
A 1st class degree would only be a very, very slight advantage - when it comes to future graduate job applications what will be far more important is the rest of your CV, your work experience, extra curriculars, your performance at interview etc... As long as you get a 2.1 you have the academic angle covered for a future legal career.
If you think you'd enjoy life at Cambridge then you should apply.
I would absolutely not make your university choices based on perceived ease or difficulty of getting a first.