Could be your feet spreading -- mine certainly have -- but it might also be the soles of your feet getting dry and cracked because of your diabetes. Flexitol is worth a go to try and fix the latter. Available from most chemists and not expensive.
Mantas are good boots and a few people do use them for summer walking -- the ones who also wear hair shirts and carry a kitchen sink with them. They're a serious winter boot and far from ideal for summer walking imo. Get a boot for the type of walking you will be doing, not something for what you might be doing a few years down the line.
If you're used to the Burma Pros, which are B0 rated, then imo it would be a mistake to go for a B1 or above rated boot for UK walking. They will be clumpy, hot and uncomfortable, and my take is they won't give you're ankle any more "support", since they're not splints. They might offer a bit more protection from knocks, but that's about it. My advice would be be just to get another pair of the Burma Pros -- they're too heavy and clumpy for my taste, but you seem to like them and that's what's important.
My old Rohan bucket hat was actually pretty cheap, considering I picked it up in a sale for a few £s and it lasted at least 15 years before I lost it -- or it got eaten by an elephant while I wasn't looking.