The British pound is still exactly the same currency that it was prior to decimalization, so very old Bank of England notes are still just as valid now as when they were first written back in 1725. They're quite proud of their reputation for never reneging on their "promise to pay the bearer".
As for coins, the situation with certain predecimal small silver coins is ambiguous. The legislation that revalued Maundy Money to be in new pence, rather than old pence, was retrospective, so all surviving Maundy coins have in theory had their nominal face values increased by 240%. This also theoretically applies to non-Maundy threepence coins with he same design and composition as Maundy threepences. Of course, the silver value alone is far above this nominal face value, so it rarely becomes an issue as very few people will be taking their "predecimal" Maundy coins down to the shops or the bank for face value.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis