You don't need to take extra adaptors wor... It is same as Hong Kong, rectangular blade plug.
The famous outlet close to London is Biscester. Here you can find Fred Perry, Burberry, Polo, Folli Folli, Clarks, Dior, Bally, Anya Hindmarch, Tods, CK, Ted Baker...... too many. My comment: BANKRUPT!!! It may not be so attractive now as the pounds are very expensive, reaching the historical high. Visit
http://www.bicestervillage.com/Bicester/home.asp?lan=en
You can take a train from London Marylebone or Birmingham Moor Street to Bicester North station, then the shuttle bus to Bicester Village.
About the local markets, recommendations include: Spitalfields market, Sunday (up) market (close to Shoreditch or Aldgate East tube station), Borough market (for food, close to London Bridge tube station)
The souvenirs from Manchester... of cos is everything about Manchester United lar...