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'