For David Haselgrove, it was a battle each day to get out of bed, then another struggle to put on his socks. Stairs were often impossible, and the pain made him tetchy and difficult to live with.

But when he sought medical help for his arthritis, Mr. Haselgrove was told the wait for a specialist consultation was more than two years. It might be another two years before surgery.

“If I wasn’t the person I am, I would have been losing the will to live because the pain takes over your life,” said Mr. Haselgrove, 71, who is now fully mobile after a successful hip replacement.

His recovery has nothing to do with Britain’s National Health Service.

Instead, Mr. Haselgrove, who ran several small businesses during his working life, flew to a clinic in Lithuania to have surgery, becoming one of a growing number of Britons who have dipped into their own pockets to pay for procedures to which they are entitled free on the N.H.S.

Free, universal health care — funded from general taxation and payroll deductions — is the founding principle of the National Health Service, one of Britain’s most revered institutions and the most enduring part of the welfare state that the country set out to build after the suffering of World War II.

But it is a promise that takes ever longer to fulfill.

After the financial crisis, the Conservative-led coalition government, elected in 2010, embarked on a period of austerity during which health spending failed to keep pace with the needs of an aging population.