A drone capable of lifting that I can certainly think of a few uses for. A few questions come to mind.
Brimstone's primary reason for existence was to replace the Belouga Cluster Bomb in the role of smashing up armoured-columns. The idea was it was a PGM that you could salvo fire at multiple laser designations at once - thus smashing the whole column in one strike (which the Belouga could very definitely do) rather than having to make multiple passes.
A 300kg capable drone will take 2, maybe 3 Brimstones at maximum. At which point the point the multiple launch capability is rather lost. The (large) reaper drone has been able to carry similar for years (often operating with a couple of Hellfires, the US-made predecessor of the UK-ian Brimstone, and basically the same packaging).
What would make more sense to me would be to carry one of those Hydra rocket pack conversions with laser guidance compatibility on the drone platform.