Is there a nation ,outside of the u.s. that could conceivably invade the u.k. Without a years notice?...No
Are there nations that may fk around with our assets abroad.. Possibly.
For many decades now, it does not take an invasion to seriously mess up if not outright destroy another nation. Witness Japan way back in 1945.
What is needed is a serious assessment of the THREATS, which includes far far more than an "invasion." And for an island nation like the UK, that includes strangulation by blocking the sea lanes.