The one in Croydon, I think. It is quite easy to drive to and if you don't have a car there is a tram which stops nearby and you can use their home delivery service whereby you buy your goods and take it to a desk after the tills. They will deliver to SE1, the price varies with quantity.
Also, you might be interested to know that they now deliver much of their range - you can order online (www.ikea.co.uk ). We did this and the service was good although we had to wait a while for a delivery slot (3 weeks or so).
We've driven to the Edmonton one from Tower Bridge (in a Streetcar). It took about an hour each way on a weekday and, of course, we got sucked into the Ikea vortex and had to extend the Streetcar booking by several hours :-)