If I was going to use another spirit in Christmas cake, instead of brandy, I'd choose rum. (After all, many supermarkets sell both brandy sauce and rum sauce to go with Christmas pudding, so they seem to be naturally interchangeable).
Whisky wouldn't be my own choice. However there are recipes which use it, e.g.
http://www.bbcgoodfood.com/recipes/2607642/make-and-mature-christmas-cake
so it's certainly a possibility.
Brandy needn't cost you a tenner though. Asda can sell you a half bottle of their own VSOP stuff for six quid.