Actually... Now I'm thinking I probably won't swap Manchester for KCL. I think if I'm good enough to get an offer from KCL, there's a good chance I would also get offers from Durham/UCL, which I prefer more than KCL. It's unlikely that I'd get a situation where KCL gives me an offer but Durham and UCL reject me (based on Which? which says that KCL is significantly more competitive than Durham and pretty much just as competitive as UCL). So really applying to KCL is pointless because I'd rather just go to those other unis. And besides, even if it did turn out that KCL gave me an offer but Durham and UCL reject me, I'd still find it hard to firm KCL over Bristol; I don't know if forking out for London is worth it for KCL, and tbh I just prefer Bristol more. And there's no guarantee that Bristol will give me an offer after all, so it's a good idea to have another 'safe' choice (Manchester) in case I don't get an offer from them.
tl;dr: no point applying to KCL because if they accepted me I'd be likely to also get offers from Durham/UCL, which I prefer. Also I don't think I'd firm KCL over Bristol anyway.