I am also researching a COFFEY ancestor in Rifle Brigade, mine a Bartholomew but born in Kinsale CORK. I think your William is in 1851 Census of Channel Islands as follows;
William COFFEE age 46 Chelsea Pensioner b Ireland
Mary COFFEE age 47 b Ireland
William COFFEE age 14 b Guernsey
Caroline COFFEE age 10 b England
Thomas COFFEE age 9 b England
I can also see his family in Barracks in 1841 Wales Census as follows;
Mary COFFEY 35 b Ireland
Michael COFFEY 13
James COFFEY 8
William 5
Mary 10 Caroline 2
No doubt our two COFFEY families serving in same unit knew each other but no relatuions as far as I'm aware.
Regards
Kym EDWARDS
Email kedw4008@bigpond.net.au