Lots to sort through in this!
In no particular order:
Employers everywhere like unis they have heard. You will almost certainly return to the UK, as there is little hope of getting employment straight out of undergrad in the US. IME, in the UK there is an assumption that -except for a handful of famous names- UK education >> US. The one place you could get a boost is from summer internships: these are a much bigger part of undergrad life in the US than the UK (not counting those heading for consulting, banking or law).
UOregon is a fine school. It is entirely unclear to me that there is any part of that experience that is £50K better than what you would get at the UK unis that AAA at A level should get you into. If you want to go that route, 1) practice up on your SAT or ACT and get that score up. 2) look up ‘automatic scholarships’- quite a few state (public) universities offer them
Boston: UMass Amhert is almost a 2 hour drive from Boston- without traffic. You won’t be going there very often. Tufts is really close- and even they aren’t in and out of the city all the time. For US unis, if you want a city be IN that city, or be happy with the college community.
Beach: sigh. At this point it feels as if you are still in the ‘think of every thing you want’ stage. You have always lived at the beach. There are comparatively few colleges at which beach will be a regular part of your experience. iirc, Revere Beach is 40min by T from Boston. Even for people used to UK beaches, by October only the most determined are still swimming, and Nov-April is just plain cold. Eugene (where University of Oregon is) is an hour and a half by car from the beach- and public transport is limited. If it is that important, then the UCs (esp SD or UC-SB- less than a mile away) are your best options (though the Pacific is colder than you might expect). How important an element is that really when you are investing 4 years & $300K in an education?
School spirit & beautiful buildings =/= UCs. They are much more like UK unis than you might imagine.
There are 2 kinds of aid: need & merit. Guessing you won’t qualify for need, but you might qualify for merit at some places. Your AAA prediction + a high SAT/ACT might make you competitive for merit at places like Oxford/Emory.