Try your nearest Pound shop or tourist shop. I found quite a variety of Union Jack items for the Wills & Kate wedding in a Pound shop. They also had Union Jack bunting but I made my own - no need for a sewing machine or fabric (just access to a colour photocopier/printer, hole punch and string) - colour copy several pages of Union Jack flags 2 x A4 sheet, punch holes in them, then string the lot on. Not very robust but effective and jolly. The colours might go a bit surreal in rain though.
Pound shops and tourist shops are probably a good bet for Union flags, but having a strong French influence in my family we make a big deal of Juillet Quatorze, (Bastille Day), and drape a Tricolour from the balcony of the house, and fly one on my taxi aerial and one on my wife's car.
The last lot we bought was from a place in Tanner St. SE1, west of TBR.
Vive la France,
Mais vive la différence!