Even if not technically the cheapest or closet, Skomer is simply stunning - all the birds you seek, especially Puffins, will be bobbing about on the water mere metres from you before you even land on the island. Early in the season, when everything is to the backdrop of bluebells across the island, with Choughs overhead and seals hauled out on rocks, I think there are few places in Europe to match, let alone in Britain.
So, my advice for what it's worth, if it is only a few pounds different, take the Pembrokeshire option.
Sounds interesting. BTW Is Easter too early for these places? I guess so.
I will certainly go to Skomer one day although maybe not next year