Also in the UK, and have been doing a bit of reading up on this stuff. Definitely "not an expert" though!
I've discovered that this style of bearing can either come "open", "shielded" or "sealed". Open ones have the ball bearings exposed. Shielded have the bearings covered by a shield that's only attached to the outer bearing — these are ideal for applications where the inner ring rotates. If you were to spin the outer bearing at high speed, lubricant can escape, apparently. Sealed are supposed to be maintenance free, should keep hold of their lube, and seem to be what we want here.
I learned that on a supplier's product page for some MR63ZZ bearings made by a different manufacturer (MR63ZZ is a standardised design).
After a lot of searching, I noticed that the MR63ZZ models that are available to buy in the UK are either "open" or "shielded", rather than "sealed".
It turns out that there's also the MR63-2RS design, and they seem to have a rubber seal on each side. They're more expensive, but I think they're probably good ones to go for. If I remember I'll report back after mine arrive…
UPDATE: I just found an explanation of what "2RS" means; it's "two rubber seals". "ZZ" means "shielded on both sides". See the "Expert Advice" section here:
https://www.123bearing.co.uk/bearing-housing/deep-groove-bearing/single-row/mr63-2rs
:-)
UPDATE 2: I've just installed some MR63-2RS bearings, and they are a lot smoother. They're also a lot more heavily damped so I can't spin the ball anything like as freely as before. I'll see how I feel in a few days, but I'm thinking it's probably better making more noise and feeling a bit coarser (with the original bearings), which is no doubt why they didn't come with 2RS bearings in the first place! I might try some alternative ZZ bearings…
UPDATE 3: I'm now a few days further into the 2RS bearing experiment. I think they'll probably be staying put. The heavy damping I mentioned above seems to have reduced a bit. I'll have got a bit more used to it, but they're definitely feeling lighter than they were. I can now spin the Adept's snooker ball to cross a 1440p monitor (with virtually no acceleration), and I couldn't do that when I first put them in. And they're still feeling significantly smoother/quieter than the stock bearings.