Jack,
The problem with joining cables is that you still need to make the joint "accessible". There are plenty of discussions on what is accessible on this forum, but the consensus seems to be that buried in a wall is NOT accessible
Now, obviously you are loathe to go and buy a 50m length of 10mm cable, it is pricey stuff. (about £30 for the length you need) However, if this is a job you are doing in your own house, think of how much easier your sleep will come knowing that your kitchen won't burn down in the night. And if it is someone else's house, imagine how bad you would feel if the house burned down the day after. Even if it turned out your cable wasn't to blame, how low would you feel for those weeks before the investigation came out?
I think as Securespark says, you should put this one down to experience. You never know, you may need some 10mm cable at some point in the next few months. With the new colours coming in someone is bound to want to buy some to avoid having to sheathe all their existing cables next time they install some new cable.
As to why joining 2.5mm cables is OK and 10mm is not: I agree that at first glance it doesn't make much sense. However, 10mm is usually used for circuits that draw a LOT of current. 2.5mm is used for rings and these are circuits that draw (comparitively) little current. With your 10mm cable you really want to minimise the voltage drop. Hence one run of cable.
However, are you sure you need as much as 10mm on this? I hope an electrician will give us the proper figure here, but if you are on a 32 MCB wouldn't 6mm be plenty big enough (although it would only save £3 or so!)? The 28m run is the factor in the equation I am not sure of. Good on you for using 10mm either way, bigger cables are better.