They’re still legal tender. Check US Treasury website for more, if you’re bothered.
“You may find the embassy can help. They MIGHT be in the position to exchange valid old notes. I assume you still require a visa before you go. so ask when making contact.”
Calling US embassy’s 0898 phone line to get a baffled clerk at the other end probably won’t work out effective! UK citizens don’t require a visa for tourism – they travel under Visa Waiver Program. You don’t need to apply in advance (generally) but you may need to submit some info through your travel agent/airline.