Did someone mention a Princess? If you feel the need to mark the occasion, check out the Lego Juniors Princess Play Castle. It's down in price at Amazon (via Toybroker-UK) from £20.27 to £14.45 right now.
The set includes a castle with slide, drawbridge, tower with flags and flowers, and a grooming station. Included accessories are a food bowl, grooming brush, and royal jewel. It's all packaged in a sturdy LEGO container, too, making it really handy for storage or travel.
Out of 96 customer reviews, 65 Amazon customers have awarded this the full 5 out of 5 stars. Here's a fairly standard comment:
"This is great little Lego set in very cute plastic box... It is suggested for ages 4-7 but both of my daughters 3 & 9 love it. It is easy to built for the little one but still attractive enough to the older one as well. The set comes with good selection of standard size bricks and couple of special parts we haven't seen in other sets such as golden chains for the bridge, golden windsock, purple slide, blue jewel, hair brush and a pink magic wand. The wall with golden window has flowers and crown printed on it."
Delivery will be free if you're a Prime customer but I think you'll have to pay for delivery unless you're spending £20 or more, under Amazon's new guidelines.