The HW30 usually produces about 7ft.lbs in .177 and 8ft.lbs in .22 as standard. In my view, a practised shot will cleanly take rabbits at close to medium range with this sort of power. The rifle has a lovely, mild firing cycle making accurate shooting very possible. Go for the S version with the Rekord trigger.
I took many rabbits with my Meteor Super (of unknown power) and a FWB127 (which I later learned "only" produced 9.5ft.lbs).
If the young chap can handle a slightly heavier rifle, the HW99S would make a terrific choice. Very accurate and still very forgiving, but also produces "full UK limit" power.
The BSA Supersport and the later Lightning version are still available new. However, many would say that they don't represent such good value for money now against the two rifles mentioned above and also when compared with a fair few of the ever-improving "budget" rifles. They do make an excellent secondhand buy, though.
Many Gamo models are worthy of consideration. Cheap, most are light and most have very slight cocking effort requirement. Models like the Whisper and Varmint models can produce high energy figures in unrestricted markets. Once detuned for the UK they display very benign recoil characteristics and very low cocking effort. Triggers are not the best, though.