According to V3, a Nokia spokesperson said the launch of the Nokia Lumia 900 in the UK will be seen on May 14 (earlier date we last saw from Phones4U at April 27, though Nokia never mentioned a specific date) because of high demand in the US. We heard Nokia were trying to make Lumia 900s as fast as they can for the US market but it seems even that is not enough and now stock will be diverted perhaps to US? It could really be great demand or that production is inexorably slow.
The overwhelming demand for the Lumia 900 in the US with AT&T has, unfortunately, had a small knock-on effect on product availability in the UK
Update: As noted in the comments, the production will probably be focusing on producing Lumias for US AT&T as opposed to generic UK model, not actually shipping the LTE free ones to the US.
Cheers Viipottaja and Janne for the tip!