Sorry to hear of your loss are you looking for perennials or annuals?

Mums or Asters are just coming into season. I live in the US and here, the caretaker will remove them. You should really consult the office to find out the policy on plants. Some cemeteries here don't allow them.

Your biggest problem is the need for the plants to be watered, unless you are able to visit regularly. I would suggest that for the summer perhaps miniature sedums and for the cooler part of the year, then miniature bulbs.

Perennial geraniums more or less look after themsleves and have a long flowering period. They mostly disappear in winter when its nice to underplant bulbs such as small daffodils, chinodoxas etc which will disappear in later spring as the geraniums start to grow. Both should go on for years.
Bathgate's suggestion of chrysanthemums is good for now - you need the smaller spray kind, not the tall ones. But it might be best to move them to your own garden after they've finished so perhaps having them in pots sunk into the ground so you can easily move them when they've finished would be a good idea. If we suddenly start to have a nice summer they'd need a good soak every few days though.

welcome to GoY
sorry to hear of your loss. check with the crem as suggested as they may have a policy of planting rose bushes. one of our local ones allows roses, bearded iris and spring bulbs only. It is beautifully maintained by their own grounds men. It is a wonderful place to go. On anniversaries you can add a small posy or make a donation to its upkeep.