1. Realistically, King's is out for you as their UKCAT cut-off is typically around 700 and they use it heavily in selection; while people have got interviewed with lower scores, they generally have to be exceptional at everything else.
2. Often a med school will state on their website under selection procedures that they rank by UKCAT (eg. Southampton, Durham). You can assume that you'll be below the cut-off for these, especially as people with high UKCATs specifically apply there. If there is a floor cut-off, but not everyone above the cut-off gets interviewed, this will generally be stated too. Otherwise, many universities use UKCAT partially, so there is no strict cut off. You can get an idea of the scores acceptable using freedom of information requests (search foi a100 <uni name> and there's info on the stats of successful applicants and short listing/interview score sheets).
3. Yes, there are easier BMAT universities. Brighton, Leeds and Lancaster are the less competitive BMAT schools. For 2015 entry, Brighton's BMAT cut-off (they rank BMAT) was below the national average. Leeds use BMAT partially (from memory 10-15%)- if you have AAA at AS, you've maxed out academic score which is the main selector, though 40% of applicants have full academic score so you need a decent personal statement too. Lancaster rank on BMAT after GCSE+A level requirements have been checked, but this is a new thing so there isn't a probable cut-off yet. I imagine it would be somewhat similar to Brighton.
Don't stress too much about low UKCAT- lots of people get in with low scores, you just need to apply wisely.
BMAT universities are good, and BMAT is generally seen as easier to prepare for, so with some practice, you should be fine for some of the lower ones (or some of the higher ones!). The past papers should give a better indicator of performance than UKCAT mocks. And there are UKCAT unis still open to you- with your GCSEs, depending on which subjects the A*s are in, you are probably above Birmingham's cut-off. Cardiff only uses UKCAT as a post-interview tie-breaker, though your statement would have to be excellent to secure a place with 6A*s and 3As as top 9 GCSEs. Bristol is only 10% UKCAT, but again, excellent personal statement needed.
Good luck!