If you can’t get an appointment with a gp because it’s a weekend there’s always a walk in centre? You might have to sit and wait for an hour or two.
They’ll probably only give you cocodamol instead of codeine on its own (they genuinely seem to think everyone is addicted to painkillers) but you might get lucky depending on the Dr, and if you have the paperwork from the hospital it should make it a bit easier to get a prescription too.
You can get cocodamol over the counter as well, but the max they can give you is usually 8mg rather than 30 without a prescription. X