In the professional game, squeeze ball (posting the ball back through the legs from a kneeling position) is a common technique at the tackle area. And no wonder because it's very effective, especially for buying time when isolated. Occasionally the tackled player gets pinged for holding on or being "beaten by the ruck", but a pro team wouldn't dream of not using it.
However, the RFU is very clear about the use of squeeze ball in the junior (up to U18) game. In Appendix 2 to Regulation 15 (Age Grade Rugby), it is clearly stated and is reproduced below:
"No player shall use the technique known or referred to as “Squeezeball” and no person involved in the teaching or coaching of rugby may teach or coach to encourage the use the “Squeezeball” technique.
Note: “Squeezeball” is a technique where the ball carrier goes to ground, head forward (touching or close to the ground), irrespective of immediate contact with opponents, usually keeping parallel to the touchline, holding and protecting the ball close to the chest and, when on the ground, pushing the ball back between the legs."
Although the NMCL is defined as an U19 competition, the Committee reiterates that due to the large number of U18 players (and even U17s) in the competition, squeeze ball is not permitted in any NMCL match.