DD was, effectively, thrown into the third year of the integrated MSc in Biomedical Engineering and opted for the more bio-mechanical options. She did her major piece of group coursework with four or five West Europeans Masters students who could well have been taking the course you have seen, and who seemed to have Engineering type backgrounds. (She was concerned because their engineering skills were better, but then discovered they were delighted to have a medic in their group, because she knew a lot more about anatomy, an early lesson in the advantages of working in a diverse group with complementary skills.) Quite a lot of courses being available as options for people on different degrees, so plenty of overlap. There was also a lot of research going on, which made it feel quite cutting edge. (Later DD spent her two month elective in one of Imperials research labs, which again was cool.)
Being comfortable with maths and some programming really important. So much now seems to boil down to the ability to handle data,. It was surprising how often DD would phone her brother who studies, essentially, economic statistics to ask advice on data sets. DD was only required to have maths A level, but I see that the integrated MSc looks for maths and physics. (As it was she also had physics and electronics, both of which were useful, though the electronics was essentially covered in the pre-sessional course they gave intercalators.) I would suggest double maths and physics if he is aiming for a competitive course. (In part because most of them are likely to have general first and second years with the options coming in third or fourth years, and they will be looking for strong mathematicians. There also seemed to be a lot of biochemical stuff going on, so chemistry would be a good if he is up to a fourth A level.
A lot of Universities will offer four year degree which include a Masters. This means that you only get charged UG fees for the final year, and you are eligible for student finance for the whole course. The problem comes if you want to switch Universities for Masters, either to "trade up" or to study a more specialist area not available at your first University. Funding for Masters is a lot more difficult to obtain and some of the fees charged, especially at Imperial, are eyewatering. (£34,500 for the Masters in Management.)
And do look at a few universities. For the intercalation courses DD was surprised how much course content varied.
(DD was torn between bio-med engineering and medicine when she was applying, and opted for medicine with an intercalation. She still has the option of switching, perhaps for a PhD, though enjoys medicine so thinks she made the right choice.)