You should probably go over this plan with your US/UK tax advisors, in a joint meeting. They should be able to suggest a number of tax-efficient ways to make your investment, such as ISAs, increased pension contributions, etc. Also present at that meeting should be your investment advisor, so that the investment side of the implications of various proposals is fully discussed. Yes, it costs more than meeting each of these people one-on-one; but you are ready to make a serious investment and you should not go into it lightly.
One problem with IFAs (and US investment managers) is that they are often only licensed to sell Unit Trusts and Investment Trust (or in the US, mutual funds). This severely limits the scope of their investment advice, since obviously real estate, insurance instruments, company pensions, and direct holdings in stocks are also investments, none of which pay your IFA a dime. You therefore may want to seek not merely an IFA, but an IFA qualified to sell direct holdings in stocks and offshore- and US-based assets.
Further, you may wish to hire an IFA who is paid on a fee basis rather than a commissions basis. This will align his interests (growth in the value of your assets) with yours; rather than pushing the sale of whatever happens to pay the best rates these days. Yes, yes, I know an IFA is not supposed to do that; but there is no incentive for them NOT to do that other than sticks. With fee-based compensation, there is a carrot as well.