The United Kingdom hampered itself significantly post-war. Between misusing Marshall Plan funds by not putting enough into capital expenditures and industrial infrastructure and transportation systems, the rampant nationalizations, gutting of the aircraft industry, Rootes opting not to purchase Volkswagen(!) dirt cheap, avoiding the sterling convertability crisis of 1947, etc there's a lot of missteps that occurred. Avoiding unions going rampant (the coal miners forcing conditions that necessitated the three-day week... oomph) would also be necessary.
More immigration tends to be beneficial for economic growth, and a stronger economy would mean both more immigration and less emigration.
If the United Kingdom had over time averaged growth 0.5% higher annually on average from 1945 to today, it'd be 44% larger today. 0.75% higher annual on average growth would mean a British economy that's 72.5% larger today. 1% higher annual growth on average would mean an economy twice as large.