I see the number (top right of screen) on the Christmas appeal video is +44(0)845 838 4699

Aren't UK 0845 numbers impossible to dial from some calls providers overseas?

But, hey, why give up the chance to boost an operator's profits? It is Christmas, and they must need (sorry, want) as much as they can get.
And it is the overseas operator who pays for the uncompleted call?

Hmm. I do not have a definitive answer to your first question. I daresay that there are still errors in some countries' and networks' programming, resulting in failures to +448.
But if networks can discriminate charging to +441 (and 2 and 3) and +447 then +449 can not be far behind. Leaving aside the regulatory horrors or free access to PRS.
And we thought that short stopping had been a problem...

Last I heard there may be some overseas operators that may not connect calls to our NGNs, or if they do, its most likely they charge more than to a UK GN simply because it costs the overseas operators more to connect the call and it's hard to determine how much because it can vary.  How much more they would charge if they do carry the calls I'm not sure.

A while ago now, Ofcom did admit that some overseas operators may not allow calls to 084x/087x for this reason.  Not sure if this is still the case now however