Welcome to GoY.

They don't seem fussy as long as the soil doesn't get waterlogged.
RHS say peat free compost and doesn't specify that it needs acid conditions.
lots of advice in RHS link below.

https://www.rhs.org.uk/plants/phormium/growing-guide

As Seaburn says, Phormiums are not fussy. You can plant them anywhere in your garden except beside a pond …