I use 2 methods for tracking down Channel Islands "grid references":

1) Google Earth: All Channel Island grid refs are UTM
references, usually expressed without the UTM Grid Area
prefix of 30U. You can set Google Earth to deliver UTM
refs by going to: Tools > Options and selecting UTM
from the relevant display on offer
 

Examples:
Alderney lies entirely in (30U) WA and the other
islands in (30U) WV

Jersey, Ouaisné Common: (30U)WV595475
Alderney, Platte Saline Bay: (30U)WA569079
Guernsey, Fort Doyle: (30U)WV2358838

2) I also use the AA/OS Leisure Guide to the Channel
Islands (1987) published by AA, ISBN 0 86145 514 2 (AA ref: 53989)

This has gridded maps in it at the back and is hugely
useful.

The base maps from which these are drawn are:
OS map of the Channel Islands at 1: 50 000 scale.
 

OS/States of Jersey Official Leisure Map of Jersey at
1: 25 000
D. Mil. Survey Mapping of Guernsey at 1: 25 000
D. Mil. Survey Mapping of Alderney, Sark, Herm &
Jethou at 1: 10 000