The RFU offers a number of training courses which are applicable to referees including The Headcase Concussion Awareness Course and the Introduction to Safeguarding Course.
You will need an RFU GMS account to find and access these courses.
Go to https://gms.rfu.com/GMS/Dashboard/Organisation/464890 and log in.
Remember you need to do the Headcase course every year regardless of what level you referee at and need to do the Safeguarding course if you wish to referee any age grade rugby.