First draft ...

Menai Bridge and Bangor may be good options to look for a hotel. There's the Victoria ... this comes recommended ...
http://www.a1tourism.com/uk/victoria1.html
Not quite large enough (I'd like to think) but it could make a suitable ops centre if paired with another hotel in the town. There are quite a few others ...
http://www.hotels.uk.com/search/area...idge&region=Y2

Up near Holyhead there's the Trearddur Bay Hotel. It's large, but doesn't quote a price on the website. Not a good sign.
http://www.trearddurbayhotel.co.uk/accom.php

Beaumaris is a pretty place ... The Liverpool Arms, though it's not clear how large it is.
http://www.liverpoolarms.co.uk/
And the Bulkeley, but at a price.
http://www.bulkeleyhotel.co.uk/

I'll look further, but if anyone has experiences of hotels here, whether on or just off the island, it'd be good to hear.