The two Part IA (second year) physics papers collectively more or less represent the entirety of year 2 (ish) physics teaching at more other unis. You usually don't get a great deal of choice in modules in physics degrees till third year anyway even at other unis, due to needing to cover a large chunk of core material regardless - usually where you do, it'll just be one or two and they'll really be more like "taster" modules in subfields of physics, because you won't have the breadth and depth in the core topics to really do anything more in depth. So the Cambridge format is not necessarily a bad thing.
Note though at Cambridge, usually you will end up specialising in one or the other by third year, and probably most often by second year for physics or chemistry - I gather you
can go into Part II (third year) chemistry with only one of the options, but that it's unusual and your remaining options may be more restricted. For physics as noted since so much of the advanced physics optional content (at any uni) will depend on often quite a wide range of topics from the first two years, I'm not sure how likely it is to be able to go into that with just one of the two. So I expect in many case you'd really need to commit to either physics or chemistry by taking both options by second year.
As an aside, I think for physics and chemistry Cambridge normally have far more spaces for the MChem/MPhys year than students on the course, so the attrition factor is whether you meet the requirements to progress - which Imperial will also have (as normally if you average below a certain level, you will be required to switch to the BSc version of a course at most unis offering integrated masters courses). So I don't think this is as much a differentiating factor. They had on the natsci website somewhere the exact numbers for each course - as I recall basically all the fourth year courses in the physical sciences regime had more spaces than students normally on the third year course (I think the only ones where there were regularly far fewer spaces were HPS and systems biology, off hand?). Also you'll have pretty much the same kinds of research opportunities at Cambridge as at Imperial, so again, not really something that differentiates between the two.
I think really the major things you need to decide are:
a) Do you want to do physics, or chemistry? Invariably you have to make this decision sooner or later, and picking a course because it lets you defer that decision isn't really that great a reason to pick it in my opinion. I also don't really think it's that likely you will find your decision made in first year compared to now. So I think this is probably something you really need to think about now instead of trying to put off - even if you decide Cambridge is for you in the end! Your comment about practicals makes me think you are already leaning towards physics more, as chemistry is fundamentally a very practical oriented subject - although obviously physics is experimental as well so you will also have long practicals for physics, but they are usually not "wet" practicals like chemistry I gather so a bit different.
b) Do you want to study in London or not? You seem to really prefer the idea of living in London so that seems to incline you towards Imperial. Although it is worth noting if you just want to have
access to London, Cambridge is
very close - it's 45 minutes on the fast train, so easy to do a weekend day trip there or something. But it's not the same as living there to be fair! I'd note though the cost differential may not be
as much - living in Cambridge is very expensive, even in college subsidised rooms. Also you will have shorter terms, which means less rent paid in Cambridge but more money spent going back and forth to home (with much of your stuff as well - since storage space out of term is, I gather, quite limited). Equally though, bear in mind in London you may well end up having to live much further out from Kensington in second year and onwards, and may spend half an hour or so commuting each day anyway depending exactly where you live and how much of the commute is tube vs walking!
@Sinnoh might be able to give some more insight into physics at Imperial if you have any specific queries which might help you confirm an interest (or not) in that option.
@CheeseIsVeg may be able to offer some insight into chemistry at degree level in general if you have any specific queries about that too