FWIW ( Â£4k is higher than my thoughts usually go):
a) If budget doesn't matter, below Â£4k, you should be able to get a nice high-end Roland (e.g. LX17) or Kawai (CA9x, I believe -- the one with the active soundboard) for that much. (There are similar Yamaha's, as well.)
b) If budget _does_ matter (you'd like to spend less that Â£4k if possible), and appearance doesn't matter, I think that best "sound per dollar" is given by a high-end "stage piano" (a Roland R2000 or FP-90, or Kawai MP11se), and a good pair of powered loudspeakers. ("good" = JBL LSR308, Yamaha HS8, EV ZXA1, or similar).
Check the "Prices Paid" thread for recent costs. Roland has just come out with a new line to look at, and the LX17 may be discounted in the shops and/or online.
Keyboard, stand, and speakers, together, should be well within your budget.
I think that, as a beginner, you'd do just as well with a less-expensive setup. But it'll take you a long time to "outgrow" an MP11 with good loudspeakers. It should sound and play very well. IMHO, when you say "I need a better piano", the next step up from that is an acoustic, or maybe one of the Yamaha Avant Grands or Roland "digital grands".
PS (edit) -- take a look at this thread on some high-end Kawai's:http://forum.pianoworld.com/ubbthre...ai-ca-98-and-ca-78-sound-difference.html