National pet register
lostlabs (labs only)
lostdogsuk
animal search uk

ok so these are not officially lost dog sites but i would also suggest:

Your local freecycle groups

your local gumtree

facebook page and invite all friends (ask them to do the same)

If your council has a webpage for lost/found then get an add on there, same with any local rescue centres.

Cats protection (ok yes its cats only lol) also have lost/found on their website.

Breed clubs for your breed of dog/cat - many have websites, and have lost/found on them

specific breed rescue centres/groups - many have websites and can post a lost notice

Hope it helps someone find their pet!