1901 census has him!
Reference RG13, Piece 366, Folio 152, Page 20
128 H Block, St George the Martyr, Southwark
Sims, William H Head M 40 Labourer b New Town, Hampshire
Sims, Ann Wife F 39 b Berkshire
Sims, William Son M 7 b Guernsey
Sims, Fred Son M 3 b Guernsey
Sims, Harry Son M 5 months b Wandsworth.
There was another at least one other Sims family living on Guernsey in 1901!