I like these "when did it happen?" posts. Timing things is really hard and the exercise makes us think about what question we're really trying to ask.

The Bank of England, presumably, became a lender of last resort before they ever had to act as the lender of last resort. They became the lender of last resort whenever they were willing and able to perform that function.

However, the economy would not be aware of there being a lender of last resort immediately. So, it would not look like it had a central bank until that information spread and was believed by other market participants. Only then could the Bank of England act like a central bank.