I expect there are fountains with mermaids and there must be some near the Thames. I remembered that there are, slightly disturbingly, mermaids on Fishmongers Hall:

http://www.waymarking.com/waymarks/WMDET3_Mermaid_Merman_King_William_Street_London_UK

Better pic: http://www.flickr.com/photos/rob_sg/6212321142/

Also on the seafarers' memorial at Tower Hill:

http://www.waymarking.com/gallery/image.aspx?f=1&guid=3cb3beb7-eddb-4517-8cc5-ad0b6f852751

And Victoria Station:

http://www.speel.me.uk/sculptmonth/1207.htm

I once got a book titled Looking Up In London from the library and I found it worth a few minutes browsing.

Perhaps the monster supplies shop in Hoxton has mermaid related Items? Don't let them give you the heebie-jeebies though!