first, there is a reason you wont get it over here… dextromethorphan-hydrobromide is rare in the uk, or is combined with other stuff so you wont get nyquil… but you can replicate its effects

paracetamol for the fever, available anywhere, use upto 1000mg per dose, max 8000mg per 24 hours

1st generation antihistamine to help you sleep (choose from chlorpheniramine maleate, or my favourite, diphenhydramine, now sold as a sleep aid) i recommend 50mg of diphenhydramine or 15mg of chlorpheniramine

cough suppressant, now for this you could use codeine, its usually available combined with the paracetamol, or you could use guafenesin

alcohol, choose your favourite, a beer or two, a couple shots of whiskey or vodka, a glass of wine,
just remember to limit your alcohol intake to less than 3 units per day if your taking paracetamol.

throw some menthol crystals in a bowl of hot but not boiling water and put it near your bed and thats it… enjoy some good sleep.

and next time your in the states, bring some bloody nyquil back!