This wasn't my post over on BB it was another members but i think in this situation he wouldn't mind me copying it:
Commit to heart the 4 rules of gun safety:
The 1st Law of Gun Safety - The Gun Is Always Loaded!
The 2nd Law of Gun Safety - Never Point A Gun At Something You're Not Prepared To Destroy!
The 3rd Law of Gun Safety - Always Be Sure Of Your Target And What Is Behind It!
The 4th Law of Gun Safety - Keep Your Finger Off The Trigger Until Your Sights Are On The Target!
Practice long and carefully on paper targets before you even think about shooting at a live creature.
Save up every penny you possibly can for your rifle, then find another £50.
Buy a second-hand "springer." I can thoroughly recommend Weihrauch in your price bracket. I'd suggest an HW77 or HW80 in .177"
Make sure the rifle has been well-maintained; and not abused, neglected or butchered by some back-street "gun expert."
Buy top-quality pellets, not hardware store junk.
Iron sights will do fine to start with.
Buy a packet of 30mm "fluorescent" stick-on labels from Staples or WH Smith:
http://www.altecweb.com/home.asp?cat=OSAV32326-K&rf=frg
(30mm is a useful approximation of the size of the "vital zone" of a rabbit)
Zero your sights carefully.
Set up a target at, say, 50 feet. Using the exact kit you will use for hunting, and firing from the position you will use (prone, kneeling etc.), fire a slow, deliberate series of five shots at the target. Did they all hit the centre of the disc? Excellent. Move the target out 10 feet and try again.
As soon as even one pellet fails to hit the disc, you have discovered the maximum range at which you can shoot without taking the chance of maiming a living creature and condemning it to a slow, painful death.
Fire a series of shots at different ranges so that you can master the hold-over/hold-under for your particular rifle/pellet/hold combination.
Never overestimate your abilities: An animal can die a horrible death if you get cocky and think that you can shoot better than you are actually able to.
Remember an ancient Greek saying: "The boys throw stones at the frog in fun, but the frog dies in earnest."
Shoot safely!