I’d go for a used bike with hydro brakes at that budget.
Boardman ADV 8.9, Vitus, Pinnacle, Ribble… probably some “bigger” brands as well.
The Boardman is a particularly good bike.
Definitely ^^this^^ £900 should score you a used tiagra/rival/105 (Maybe older GRX600?) hydraulic equipped bike with an aluminium frame and carbon fork of you shop about. You won’t get that new currently.
IMO hydraulic brakes are well worth it over cable discs, upgrading after the fact is more cost and ball ache (trust me) and eventually you’ll come to recognise cable discs have their issues/limitations.
My CX bike is using cable discs, mostly on cost basis (what I had in the spares bin) and because they only need to be useful for about an hour at a time. The gravel bike see’s more year round use, longer rides and generally covers more miles, better brakes are worth it in that instance.