We have UK only wills, with the majority of our financial assets in the US. Our US financial accounts have our children as named beneficiaries which I believe takes precedence over any will, US or UK. I think the brokerage will only need copies of the death certificate along with letter from our lawyer to disperse the funds to our children.
Also, not sure of the legality of having 2 wills, each claiming to be the last will and testament.