Puppies
Cute and cuddly but high maintenance for at least the first 6 months.  House training, chewing, nipping, socialisation and obedience training, to name but a few.
 
There are many websites available to help you make the right choice and provide hints and tips on how to prepare for your new arrival, feeding, basic training etc., here a few for you to start with: RSPCA; PDSA; Blue Cross and a general web search will bring up many more for you to read.

Rescue dogs
Rescue centres strive to rehome pets that for various reasons find themselves without someone to love and care for them. Their owners may have died, may longer be able to afford them, have moved to a no-pet home or had a baby. In extreme circumstances they have been abandoned or cruelly treated. They range from puppies, just like any other cute puppy you may buy from a breeder, to older dogs which have the benefit of being housetrained and past the chewing stage.

There are many Rescue centres, some big names you will recognise:
RSPCA
Dogs Trust
Battersea Dogs home
Smaller local ones:
Wood Green (Huntingdon)
Block Fen (Peterborough)
Heathlands (Royston)
Luna Rescue (Hertfordshire)
and those that specialise in rehoming specific breeds for example Jack Russell Terriers (Bedford), Labrador Rescue (Southern England) to name but a few.