Experia Z Ultra
While it's true this is the smallest "tablet" produced by Sony, the Experia Z Ultra is a phablet with a 6.4 inch screen. Though it's only an 800 SOC, it is on KK 4.4.4 with Lollipop promised, it has a SIM slot and it's currently available for between £207 and £235 depending on where you go.
You lose 120px off the side of the screen compared to a tablet, but I bought one as a cheap developer tool a couple of months ago and overall I think it's amazing value for money. Just put a screen protector on it, it has the funny Sony coating over the glass which can scratch.