The LP50 RF is the newer improved version of the LP50. I'm sure as LP50 stocks deplete they will be replaced by LP50 RF.
Any RFD who stocks Steyrs should be able to order through who ever is the UK supplier (forgot who it is someone else here may know).
Otherwise you may be able buy from Buinger in Germany. I bought all my Feinwerkbau rifles except the 2700 from them. Though with Brexit I'm not sure if there would now be any import tax to pay now.