I like Challenger a lot... But I'd swap your hops around... Something like this...
30g Challenger @ 60 mins
15g Challenger @ 30 mins
Then Whatever with Amarillo.
You could make a best bitter. Challenger is well suited to the style.
Malts... Pale, Crystal and a dash of chocolate (or brown)
Hops... (30-40 IBU) 60 min Challenger, 30 mins Fuggles, 15 mins Goldings and 5 mins Goldings
Oh trust me as long as it tastes good I'm down with whatever comes out. I have some chocolate and some brown kicking around too. I basically just picked a style and added bits until the checkmark went green. I'm hoping it turns out