Relative long term decline was inevitable from being the workshop of the world in the 19th century.
They were doing OK.
Britain was paying its way in the world, and paying off war debt to the USA too, quite satisfactorily before joining the EEC.
If you look at the attached graph; you will see that the UK's balance of payments account was in balance before 1973.
https://www.ons.gov.uk/economy/nationalaccounts/balanceofpayments/timeseries/hbop/pnbp
It was after the UK joined the EEC that it all started to go wobbly, and the UK started running up horrendous deficits.
It wasn't doing ok hence why both Tory and Labour governments wanted in.
Look at growth statistics or productivity. We've been in steady decline since WW II even relative to our European neighbours never mind the fast expanding economies of Asia.
Sadly the EU wasn't a magic solution. Nor was the Thatcherite revolution. Nor was Blair's dream of us being a hi-tech powerhouse whilst the 3rd world did the actual building of stuff.
In the meantime our businesses continue to prioritise short-term rewards for shareholders over investing in their businesses for the longterm.