I am in and intend(ed) to ride it on fixed. I would like to know a little more about the proportion and frequency of really steep bits though. On my scale of things, Brevet Cymru / BCM type roads are very fixed friendly. Elenith is doable on fixed for most of the route so isn't too bad since the walky bits are very obiously walky and not too long. I don't really know much about Pembrokeshire terrain - if it is continually steep up and down along the coastal bits (as, e.g. Cornwall) that could be a bit more worrying.
Talking of worrying, perhaps I should forget I have entered this for a few months and leave the fretting until July.