Has anyone cycled London to Fishguard?
It seems likely, since that is exactly NCN 4 - however, I haven't found any references to people riding the length of route 4, and the CTC route database isn't giving me anything either.

For reference, I plugged Charing Cross to Fishguard into cyclestreets and it reckons a shade under 300 miles (for the balanced route). Whether that means three consecutive 100-mile days, or five 60-mile days will depend on you.

I'd take the cyclestreets routes with a grain of salt, or at least study them carefully (and compare with the sustrans route description and mapping linked above).

Oh, and I see about 2km of total ascent on that route, with what look like some steepish sections once you're into Wales.