Before you go mad with the drill and get the head off you may get a stud remover bit into the head of bolt (they are a tapered hardened bit with a LH helix on them, the harder you tighten them the harder they grip the hole and because they are LH they turn in the 'unscrew' direction of a normal RH bolt or stud).
if that don't work you are going to have to drill them out, use the same diameter drill as the bolt or up to 1mm larger and when you reach the shank of the bolt the head should just fall off - Allen bolts are high tensile steel and pretty hard to drill, so you need a good quality drill bit (the ones you get from B&Q etc are normally only OK on wood (best Chinese hardened mild steel bits) get Dormer or De-Walt from screwfix or engineering supplier).
https://www.lowes.com/pl/Screw-extra...ols/4294607735
this is just an example, screwfix or similar may do them, Irwin are a pretty good quality.