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a .22 and a .223 are quite different in size. A .223's case is 45mm long, while a .22lr is merely 10.5. You would instantly be able to tell the difference if you were to hold both in your hand. The difference in energy is quite different as well, the .223 being stronger by several magnitudes. While the .223 is greatly stronger than a .22 (and an excellent varmint cartridge, by the way) it still isn't that sharp in recoil, and has a relatively flat trajectory which should help with those long shots. Other major differences are that .22's are rim fire cartridges, and .223's are center fires, meaning that the .223 will be more reliable as well, since .22's occasionally have difficulties with primers, or are just finicky in general. Perhaps it would help you get a better idea if you were to know that a .223 is essentially the same cartridge as the NATO standard 5.56X45mm, but just a touch less powerful.

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If your intention is to eat the rabbits, then by no means would you want to use a .223. Its a rather high velocity cartridge and could very well ruin it for consumption purposes. It won't do anything so dramatic as cause it to explode or anything like that, but it would waste more meat than you would desire. For rabbit hunting, a .22lr is more than sufficient. As a matter of fact, that just might be the ideal cartridge for you, since your not particularly hunting anything very large. A shotgun is nice as well, but i think the .22lr is definitely right up your alley. Not only is the cartridge sufficient, but its rather inexpensive (here in the United States at least) and therefore it will give you the ability to practice with it at a fraction of the cost that it would take to use a shotgun or center fire cartridge. Just make sure you load it properly for the intended game, something that i think (but you should double check of course) would require hollow points for maximum efficiency. take care, and if you have any additional questions there are scores of people at the website http://www.thehighroad.org that would be more than happy to answer any questions you might have. They even have a hunting forum, and other people from the UK who could possibly even help you with legal issues. Happy Hunting.

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The projectile size is exactly the same the only difference is that there is more gunpowder in the .223 round.

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Usually .22 refers to .22 Long Rifle, which is a tiny shoulder less rim-fire cartridge, useful for hunting rabbits and similarly sized game. The .223 round is a shouldered center-fire cartridge that is substantially larger and has substantially more powder behind it. The latter can be used for hunting coyotes and such, but I wouldn't use it for deer unless the deer is rather small.