You can get saline drops and vapour rub (for chest and throat) suitable for 3months+ over the counter at decent pharmacy (Boots, or most supermarkets). If you not happy with GP then seek further advice. Having a baby with a heavy cold is horrible and worrying, but as long as her temperature is under control (not elevated and controlled by calpol) then if it was me (and it has been) I would use simplest methods available for managing blocked nose and chestiness. Her sleep will be restless, but then no-one can sleep when they're congested.
I am not a doctor, so please don't rely on just what I say. There is always NHS direct on 0845 46 47 - they may be able to advise you properly.
Snuffle babe is the baby rub, much milder than Vicks! Smells nice, but no idea if its effective at all - I used/use it on dd2, but suspect its for my own benefit and she likes the smell
I did use Otrivine when she was under 2, but then again she was a bit of an extreme case - unable to breathe properly due to huge tonsils, adenoids and laryngomalacia so having a blocked nose due to a cold was extra stress, and she had severe sleep anpnoea. We were told to use Otrivine, Medised, whatever helped get her through till surgery, basically....not just a chesty cold.
Now she's over 2 (nearly 3) I use Otrivine sparingly if she's very blocked and asks me to (rare!).
the company that makes them will not be liable if DC has any side effects. Your doc will have to take that on the chin as the product does not have a licence to use in under 2. They are only to be used short term(ie 1 week)...even in an adult as you get a rebound reaction with long term usage.
Saline nasal drops and a suction mucus remover are good.
I dont think you can use Vicks vapour rub in under 2 either unless its the infant balsam. Havent got the pack in front of me ,but check.