The short answer
By several important measures Britain is in relative decline, though the picture is mixed and contested. Living standards have stagnated, the state struggles to deliver, but the country remains wealthy by global standards. Whether that adds up to decline depends on what you measure.
The economic case
Wages and living standards have barely grown for around fifteen years, a stretch some economists call the worst since the Napoleonic era. Public debt is close to £3tn, near the size of the whole economy. On some measures large parts of the country are now poorer than the poorest US states. The figures, with their disputes, are on the evidence page.
The public realm
Beyond the numbers, the state visibly struggles to do basic things: to build, to control the border, to run services within a budget. Those failures are traced on the human cost, small boats and why Britain can't build.
The case that decline is overstated
A fair account has to note the other side. Britain is still among the richest countries on earth, a scientific and cultural power, with a large economy and strong institutions. Some argue declinism is a recurring national mood rather than a measured fact, and that many indicators are stable or improving.
Where the thesis fits
Starkey's contribution is not to prove decline but to explain it. He argues the cause is constitutional, a shift from politics to law that stops elected government acting. That claim is argued, not assumed, on cause and effect.