This is highly speculative but I'm curious. I'm currently doing a JD at a top 6 school in the US. Grades are top 5% after the first year but two more to go so who knows how that will change. I did my undergrad at an Ivy league over here(graduated approx top 35% of my class, cum laude). Originally British, came here for college and stayed ever since. Have offer to go to US law firm next summer for the summer between my 2nd and 3rd year. Hopefully will turn into full time offer. So assuming I work for a couple of years in the US doing that, how likely would I be to get into Oxford BCL or Cambridge LLM, and if I would have a shot how high do I need to keep my grades these next two years? Any input would be much appreciated.

Probably impossible to say at this point. If you apply straight from law school (which I realize you say you will not), you're going to get a conditional offer, the condition being your final cumulative GPA. The schools you attended will help no doubt, but in my experience it is your performance that matters most. Work experience will also weigh heavily in your favor, but to go to either Cambridge or Oxford, you've got to have a pretty solid record, both in terms of your academic performance over the next two years and in terms of where you work.