Leggings are allowed
The RFU's official Newsletter Touchline has stated that Leggings are allowed in certain circumstances.
The item stated that:
'BASE LAYER REGULATIONS
Following enquiries from coaches and parents, the RFU can confirm that it is permissible under RFU Regulations for young players, up to Under 18s, to wear base layer stockings or tights in cold weather, in accordance with IRB Regulation 12.
This regulation states particular thicknesses. However, parents can assume that most common brands will comply, although players are permitted to wear them subject to the assessment of the match referee that the items are safe and pose no risk of injury to the wearer or other participants.''''
This is contrary to the Law 4, and Law Clarification No. 5 of 2006 which defines underwear, but be aware, Whilst this new RFU ruling has published in Touchline, it has not yet made it to the RFU Regulations, so the authority of it isn't entirely clear.
There is also no guidance on what temperature is classed as cold, or if cold is referenced to the player or the referee.
The specific Newsletter can be found here and the article is on the middle right of page 3.