For something just like a real piano, Clavinova etc. are brilliant. Well worth the money. Roland, Casio also make similar. They are more expensive than a cheap piano, but are far nicer to play than a cheap piano (and the bonus of being able to wear headphones). They can cost quite a lot (up to thousands).
If you want a cheaper thing, there are weighted key keyboards (make sure you get one with speakers & sounds built in, there are also some that require external speakers or sound generators), which don’t have the fancy stands etc. but take up a lot less space, and have roughly the same action / playing feel as a big digital piano. You’d be looking at about £300 minimum I think.
If you really want to not spend much at all, I’d either pick up a cheap Yamaha keyboard – the only thing I’d worry about is full sized keys. Velocity sensitive keys would be nice too.
Alternatively, if you have access to a van, and the space for it, not very good pianos are often jolly cheap or free (budget £50 for tuning once you get it, check ebay or gumtree). We gave away the one I originally learnt on to a guy with a van for free. You can learn the basics of piano playing on any old pub piano.
Joe