For a pootling beater, I'd say a 70s/80s/90s steel bike from almost any reputable maker (ideally a Reynolds 531 one). The famous auction site has many, often at very low prices - I've bought two seriously nice 531 frames over the past six months for less than the price of a couple of top-end spokes, and I've seen many decent whole bikes selling for silly money.
You'll almost certainly have to do a basic service at least - new cables, brake blocks, tubes (and maybe tyres), lubrication, probably some bearings, possibly chain. But the bits you'd need to get are also abundant on that same auction site.
And you'll end up with a bike made from the most resilient and long-lasting frame material ever invented, designed to last forever (with appropriate maintenance) and not just until next year's model comes around.
Alan