Heya,
I done the MRes in Biochemical Research at Imperial last year, and along with two other people from that course, I am now doing a Ph.D at Cambridge. Overall I would say that the vast majority my class of around 20 are currently doing Ph.D's (I guess over 75%); with a large proportion (around 25%) staying at Imperial for their Ph.D. You can attend any lectures you want while you're at the university, so you can incorporate additional study if you wish. However 4 months is a very short period to obtain the expected level of data, so you may not have time. The focus on research may be a benefit because it means that you are very well prepared for experimental work and it is recognised as equivalent to a 2 year masters in Europe.
I would recommend this MRes, however, all of the biochem funding competitions at Imperial are for 1+3 year Ph.D's, so if you do an MRes there, you become illegible for these competitions. Please note from the above stats that about 10% were on the 1+3 program. I hope this helps!
In regards to work visa, you would have no problem in getting a visa for a Ph.D and then post-doc. You would however have problems getting visas for 'normal' jobs.