If you had the cats cremated, or if you have some hair of them, there are lovely cat's paw pendants to keep it in, and there are garden ornaments, too.
In the Netherlands there are several webshops selling them, and you could also have a ceramic artist make a statue of your own cats.
I have statues some of mine, both of living cats and rainbow cats. I order them from Irene Maas in Alkmaar, but there must be artists in the UK who can make things like this, too.