Unfortunately, I'd have to agree with the others. I've always wanted a 300 H & H. I was going to ream out a Ruger No 1 in 30-06, mainly because I couldn't easily find a bolt action long enough to feed that long case, it can be done with some factory actions, but does require a lot of metal removal.
Even over here, finding brass for a 300 H & H isn't easy , its availability is very hit and miss.
I'm not trying to rain on your parade, but a 300 WM or 300 WSM are excellent cartridges as are the other calibres mentioned above and are readily available.
That being said, if I came across British built sporter in 300 H & H I'd buy it in a heartbeat, they are very very cool. I'd worry about brass afterwards.
I guess it boils down to how bad do you want a 300 H & H
AB