A local (to the graveyard) florist may well for a small sum keep the headstone clean, remove any old flowers, and place any new ones, according to the instructions (and payment) you leave them. A little research should tell you what you want to know.
Find out how long the lease is on your chosen plot, if it's 100 years then I'm not sure what you could do about retaining that, but you could instruct a solicitor to renew if the lease period is shorter.