For my budget, should I be looking for a carbon frame?
I could be tempted by a 0% BNPL deal, then pay it off after Christmas, not sure how that would sit with the accountant though.
Depends...
With your budget if you didn't mind what you rode geometry/brand wise then you could get a good carbon setup, something Cannondale, Giant, Canyon with 105. Or more 'no-name' brands like Verenti, PX, Eastway etc you'd get lighter carbon and probably Ultegra.
If you were more conscious about the name/brand/geometry/aero then you may struggle to get a decent groupset and carbon.
Although saying that there's nothing wrong with quality aluminium, my Giant Defy 1 2015 was lighter than many carbon bikes for less than £900.