Berkshire is a reasonably well-recorded county, and while micro-moths are added to the county list on a fairly regular basis, a new macro species is a much rarer event, so two new species in one year is cause for celebration.
It's fun to speculate on where new species have come from, although it's rarely possible to be certain. The most likely explanation for Marc's Crescent Dart is that it dispersed (or was caught in strong winds) from one of its coastal colonies, although migration from the continent probably can't be ruled out. Crescent Dart seems unlikely to become a Berkshire resident species.
Mark's Oak Rustic may also have been a migrant from over the channel, but it is established in Hampshire and some of the other south coastal counties, so this could also be an unusually long-distance dispersal from there, or perhaps it is the start of a more significant expansion of range inland. Time will tell! It feeds on Evergreen Oak, which is planted quite widely, so there is certainly the chance that it will become a familiar species in the future.
UPDATE: Mike Wall tells me that Oak Rustic has been recorded at Pamber Forest this year, and Steve Nash says that it has also been found near Guildford in the last week or two, so it does seem to be creeping inland.
Photos of Mark Calway's Oak Rustic, taken by Martin Harvey