Sling for the first 3-6 months (depending on size) – far better than a pram.
Then small foldable pushchair (Petit Star Zia or Quinny Zap, both £120) for town use and an optionally something chunkier for outdoor/longer walk duty. We got a Maxi Cosi Mura for half price £150, which allows you to have the baby facing you – a good thing.
Don't get sucked into the Bugaboo/Phil and Ted thing, it's just not worth it.
Chariot are absolutely frigging amazing. You don't have to actually push them on the flat, they just roll along and you control them with a finger – seriously. And it's a bike trailer 🙂 Cougar is the most cost effective one I reckon.
Re Isofix, the Maxi Cosi one can be clicked into its base OR you can wrap a seatbelt around it old skool style, so works in taxis etc.
But best advice: get a sling first, then take your time figuring out what you want as you get an idea of what's needed. Don't rush out and spend thousands before the baby arrives!