In reply to deacondeacon:
Seeing as you've had some pretty brief answers - what Deacon has said is basically it, but good on you for asking! Sandstone (as found in Northumberland, to the south-east of London, and in the Churnet Valley in the south Peak) is much more fragile and needs time to dry out, whereas grit has a hard crust that means that if it's dry to the touch it's good to go. If you end up bouldering at Burbage South Valley, say (which is a really good place to go for a first outdoor bouldering session), there are some problems with footholds where the crust has been eroded away, and climbing on those shortly after rain will really exacerbate that damage, but those are quite obvious (big golden, sandy scoops); beyond that, as long as you make sure your shoes are clean and dry before you pull on you're being as conscientious as you can be!