I've had a single report of White Admiral from Saturday. Your's is the first swf I've heard of. Brimstones are new brood I think, as are small torts and peacocks (very few of these as yet).
Reported so far this year:
Dingy Skipper, Grizzled Skipper, Large Skipper, Small Skipper, Large, Small
and Green-veined Whites, Orangetip, Brimstone, Swallowtail, Small Copper,
Common Blue, Holly Blue, Silver-studded Blue, Brown Argus, Green Hairstreak,
Dark-green and silver-washed frits, Small and Yellow-legged Torts,Red Admiral,
White Admiral, Comma, Peacock, Painted Lady, Speckled Wood,
Meadow Brown, Ringlet, Wall Brown and Small Heath.
Probably flying but not yet reported are essex skipper, White-letter and purple hairstreaks, and gatekeeper. Shortly out will be grayling and chalkhill blue.