I guess you have used Google to search? I found this:
HOME OF THE BAC 1-11 ON THE WEB
Enter site, go to 'COUNTRY GUIDE TO OPERATORS', then 'UNITED KINGDOM' and scroll down about two fifths of the page to find
'British Island Airways, Ltd. - (KD, IV & UK/BIA)'
That lists all the BAC 1-11s they used. That should narrow it down. There are also photos of some of these aircraft dotted around - here for example:
British Island Airways Aviation Photos - Planespotters.net Just Aviation