It seems like there's a gap in the area for a weapon that size, looking at the RAF inventory there's nothing in that region, but i'm still a bit unsure as to if it'd be worth it. Brimstone is by and large a weapon for targetting vehicles of all descriptions and is well known for being a low collatoral damage weapon.
Still, for the RAF the F-35B will initially have Paveway IVs which considering it's going to be "the backbone of the RAF's Air-to-Surface bombing capability", it's good we're getting it right off the bat.
The future looks promising with the F35, ASRAAM, AMRAAM/Meteor, Paveway IV, Brimstone 2, Storm Shadow, SPEAR 3. Quite a wide target window, then if weapons like Paveway III - but not II as IV will end up replacing II and Enhanced II - it'll be quite a strike aircraft (I'd still like an AShM though . . . )