what refrigerant to use?

This mainly applies to the UK because our refrigerant Price's are going into orbit,had a look at an installation that my friend is going to undertake and he asked my advice,its second hand equipment the customer was using until 12 months ago we have been told it was on 408a but we are not 100% but its on a freezer room and the condensing unit is a ten horse prestcold R body any recommendations of which gas to use as 404a is not an option anymore.

Not tried but I expect one of the recommended replacement for R404A, R448A or R449A.

Have been converting R22 milk tank systems to R407F, it doesn't work as well imo, plus it can struggle in higher ambients if the condenser capacity is marginal.

POE oil etc would be a good idea if its still on mineral oil.