From what I can find, if what you saw was in fact a Pileated Woodpecker, it would be not only the first record in Great Britain, but the first record in the entire Western Palearctic.
If it was a Pileated Woodpecker, it would almost certainly be an escapee from a zoo or aviary, and not a vagrant.
I find it odd that, if it is a Pileated Woodpecker, it has gone unreported until now. Unless you are in a very isolated area, or the bird has just arrived, someone would have seen it. A Pileated Woodpecker is a hard bird to miss; they are big and loud.
A picture, no matter how poor, would be very useful. New birders are often absolutely sure of what they have seen, until you point out to them what they have actually seen.