Technically, my paintings of Lego figures standing on maps are derivative of the mapping data. This means that they might infringe the copyright on that mapping data. In the UK, the Crown owns the mapping data - this is generally collected and administered by the Ordnance Survey, who also administer the copyright on the data.
One of my friends pointed out my potential copyright infringement, and suggested that I contact the Ordnance Survey with examples of my work. I did so, and have received permission from them to use the mapping data "in this format" without incurring Royalty Fees.
Expect to see a one or two more Lego figures on maps next week!