I recently had a trawl on the internet of processing services in the UK and can't recall any who offer a process and mount service for 120 - only develop and sleeve. I think your best bet might be to buy a mounting press and some mounts and do it yourself. If anyone else knows someone who does offer mounting, I'll stand corrected. At least if you do it yourself you can make sure they are mounted square and you only need to mount the frames you want to.