I just noticed that reminders are working in the UK now. I can say something like "Alexa, remind me to <whatever> at <date and time>" and get a reminder. However, I cannot find any way to delete completed reminders from the Alexa app once I have been reminded (they appear in a Completed Reminders section, but there is no delete button there).
Named timers still don't appear to have been made available here as far as I can tell.