Congratulations on your admits! Sorry for a late reply. I took the weekend off
IB is a formidable industry/sector to get into. It is not as crazy of a focus as 10-15 years ago but it still requires grit and effort and networking. Is there actually much IB happening in London? (I simply don't know). I can imagine London has been in an uncertain situation for a number of years now and that's not conducive to hiring.
Tuck is solid in terms of the program, recruiting, and tight alum base. With all of that, Tuck is a consulting school (many US schools are these days) but it does place 15-20% into Financial Services (depends if you count roles - 15% or Industry - 20%). Looks like you have done quite a bit of research and put in your time. I think ideally you want to connect with the Finance club/group at Tuck/LBS and figure out how much help/recruiting they are seeing on campus and if having an IB goal is realistic and what do they think about it. I would be direct. No sense to beat around the bush but don't pressure them. I can of course see Tuckies being very proud of their program - that's how they are
I think it is good you have strong brands on your resume. You can use those to open the doors for you and provide you with internship/interview opportunities and likely would not need to leverage additional internships during the year.
P.S. I would figure out which country you want to live/work in the long term (there is a difference) and overlay that with the current student conversations and see which looks more attractive as a package? Also, do you have any support network in either continent? EU or North America? perhaps some distant family or friends? It helps to have someone who can lend a hand and be a safety net.
Good luck with your LBS decision - did you hear back?