Mature Metal discussion

We’ve all heard that things tend to shrink after being locked for a long time. However, my Mature Metal jailbird is getting looser than I prefer, and I find myself needing to tweak it more often. The main issue is the expense of returning it to the USA, which will include postage, export/import fees, and potentially adjustment costs if they are willing to handle them. Does anyone in the UK know an approved shop that can perform these adjustments? Can they modify both the cage length and the base ring diameter? The current width works well for me. I’d really appreciate any assistance.