For former resident aliens of U.S.- 401K distributions sourced to U.S. job received when individual is resident of Great Britain (country of residence)- is that taxable at time of receipt (after U.S. residency ended). Are there any exclusions?

Almost all distributions to UK residents can somehow be excluded from US tax under the US-UK tax treaty. If the dollar amount is large, it is worth retaining an expert in this area.