I can't think of a way, though Scotty knows the loopholes better than I do
With a custom recovery you could back up everything else, and if necessary you could fastboot flash a stock recovery image if you can't get a backup. I found a few examples available for download at xda-developers - can't guarantee they would be identical to the one on your phone, though I suspect the stock recovery doesn't vary much, and some UK users have made them work. So this would probably work, though I've not tried it.
The other way is to use an RUU, but that will do a factory reset in the process, and it would need to be a 4.2.2 RUU if you are S-On. In any case I've not found a Three UK RUU yet
Of course, if the worry if the custom recovery not working, in that case you can just flash a different custom recovery (TWRP or ClockWorkMod).
So the chances of being stuck without a recovery at all are slim, but it would be good to have a way of returning to stock if needed. In the absence of an RUU I think you could probably do that by restoring a backup of everything else and then flashing a stock recovery, though the bootloader would still indicate that the phone had been modified. An RUU would be cleaner for that purpose.