Sportsmatch do a couple of adjustable mounts for 30mm tubes, ATP61 (elevation only adjustable) and ATP66 (fully adj). Both are 2 piece mounts though, seeing as you have an HW100 a one piece would be preferable to prevent any breech flex (a potential issue on the HW100). They have a 1 piece adjustable in the pipeline, under "new products", but I've not seen them released yet.

BKL have a 1 piece adjustable, not cheap at around £60, but they are excellent mounts which centre well on the rail owing to the clamping method they employ.

You'll need to check any of the above for clearance/height.

I'd try just cleaning the barrel first and do some pellet testing, before honing the bore. HTH