If the UK is representative of other EU countries then it will be almost impossible to open new bank or brokerage account unless you are a resident there.
I don’t believe there are any special things you have to do to inform the Treasury of large bank to bank transfers when moving money overseas. We have moved hundreds of thousands in 2016/17 to buy 2 houses in England. The information you provide to your US bank to show where the money is coming from will be reported by them to the Treasury, and the FBAR reports you file yourself to the Treasury will also show where the money ended up.
If you carry more than $10k cash out of the USA it needs to be declared ahead of time.
Alan, did you use direct bank to bank transfers or did you use a service like Transferwise?