Those 25mm size axles, hubs & related componets are made for Cadet karts. Cadet karts are miniature sized racekarts made for children. I think you'd be much better off in the long run using stronger 30mm. (or preferably 40mm.) diameter axles hubs and related componets.
Here in the states, its common for race kart shops to sell Axles, wheel hubs, rims & related componets in your choice of metric or U.S./Inch-size bolt patterns & center hole sizes. I just spent some time researching axles, wheel hubs, rims and related componets at serveral UK racekart part websites and noticed they only sell metric sized componets (no USA inch-size listed). So that's why none of these UK parts shops give any detailed dimensions of their componets, because all of their wheel hubs will perfectly fit all of their rims.
So while I still think it's a good idea to email or call your local parts suppliers to confirm fitment compatibility, I've got a feeling that most all of the 3-bolt racekart wheel hubs sold in the UK, (that are made for 30mm. or 40mm. axles) will perfectly fit those JET rims you wanted to buy; and I'm also fairly certain those rims have a 50mm. center bore size.