what are my chances of recieving an internship from JP morgan, morgan stanley?

I do a biomed degree in my second year applying for internships i go to Southampton Uni, my friend did recieve a Morgan Stanley internship as a summer analyst with the same course and Uni as me.
but either way after RECIEVING an internship from these big interns what are the chances they actually offer me a grad job, and would a year in industry be more helpful?

Chances of getting a BB IB = low. Got to remember that the chances are low for everyone, even if you go to a very good uni doing econ/STEM.
If you get an internship in a team you like, the chances of converting to a full-time grad offer is usually fairly high. Nowadays the bulk of graduate positions in IB are given to candidates who've done internships with that bank, though this can make it tricky if you don't happen to get an internship that you want to covert into a grad offer (or you simply just don't get IB internship offers).
There's no objective answer on whether a summer internship vs a placement year is inherently more helpful than the other. It simply depends on what offers you trade-off when deciding what to do. Usually there are more applicants for summer intern roles, but there are also more places up for grabs, and they tend to be focused in the sort of areas where most people applying want to work (e.g. M&A, Sales & Trading, quant, research, asset management, etc).

I’m currently doing a degree in biomed do you think I should switch in the first semester to business management? Would that increase my chances of working at these firms?

I don't think it would increase your chances, banks place a relatively small weight on the subject you're studying (unless the role you're applying for requires specific subjects for obvious reasons) - the uni you go to is more important than what you study there.
In any case, I'd probably argue that biomed looks better than business management, it also allows you to apply to STEM positions too.

I was thinking of switching from biomed to cardiac physiology (healthcare science) but biomed has more career prospects.

Okay?

Do you think that would lower my chances of going into a career in finance?

As I said earlier, banks care much more about the university (along with some other things) rather than the course you're studying. So given that, what do you reckon the answer is likely to be?

I get your point..
But a cardiac science degree to banking is almost unheard of - and some internship you’d probably not be available to apply to