Q:

Which sport has closest tackling rules to (BQC) Quidditch? 

Our lovely quidditch team needs to get better at tackling if we want to win any games.  We thought we might be able to learn a thing or two from players of other sports, only most other sports seem to tackle quite differently to us.  
Obviously no other sport involves riding a broom, and as such most other are going to be fully two handed, so I am just looking for the closest common (in the UK) sport that we might learn from.  
The rules of a BQC quidditch tackle (same as US quidditch) are best described in the rulebook, however in brief:

Tackle below the shoulders and above the knees. 
Initiate contact from the front. 
You may not use both hands to wrap round an opponent. 
No shoulder barges. 

(Its also generally considered polite not to skewer your opponent with your broom, though interestingly I don't see that in the rules anywhere.) 
Any recommendations are welcome, 
Many thanks. 

A:

Tackling in Australian rules football sounds very similar.  From the relevant Wikipedia article on Australian rules football:

Opposition players may bump or tackle the player to obtain the ball and, when tackled, the player must dispose of the ball cleanly or risk being penalised for holding the ball.  The ball carrier may only be tackled between the shoulders and knees.  If the opposition player forcefully contacts a player in the back while performing a tackle, the opposition player will be penalised for a push in the back.  If the opposition tackles the player with possession below the knees (a low tackle or a trip) or above the shoulders (a high tackle), the team with possession of the football gets a free kick. 

Furthermore, in the article that covers 'bumping':

Charging a player is not a legal bump and is penalised with a free kick and can be reported, regardless of whether the ball is within five metres or not. 

This seems to cover:

Tackle below the shoulders and above the knees. 
Initiate contact from the front. 
No shoulder barges. 

However, in Australian rules football, players do most definitely use two arms to wrap and prevent the ball from being released, which clashes with point three in your requirements.