If its a 20 acre plot your on and its for rabbits mainly, i would reckon a .22 rimmy would be ample sufficient. If youve got more, then a 17hmr seems to be a popular choice for longer range shots.
Im no expert by a long way, but i would doubt a flo will give a first timer a licence for all 4 rimmies, then again who knows, each police force is different.
The which is safer , 22 or 17 is a bit of a hot potato, with pros and cons for both, but your right that once you get one on a licence, it costs to get another added. If youve only shot an airgun a 22 rimmy would be my choice.