Most fledgling (i.e. feathered) young birds are best left for their parents to look after, but young swifts are an exception. Swifts cannot take off from the ground and young birds who fall out of the nest or crash-land on their first flight do need help.
Swifts have a very characteristic rounded face with tiny, but wide-gaping beak so they are easily identified: Action for Swifts have several good photos on their website. There is a list of swift rehabilitators on the Swift Conservation website and they may be able to help or advise if you find a grounded Swift.
If a specialist Swift organisation cannot be contacted, the RSPCA will do their best to collect and rehabilitate swifts - call the national helpline on 0300 1234 999. Make sure you explain to the person answering the phone that the bird you have is definitely a Swift and not any other species.
The recent bouts of torrential rain seem to be causing problems for Swifts, either because they're being beaten to the ground by the sheer force of water or because water rushing along house gutters is causing damage to nests.