Good to hear that Shane is getting plenty of business, I've never seen one of his saws in the flesh but the pictures and reviews suggest they are top notch. I read your OP and you say you are not interested in second hand so feel free to ignore me, but the fact remains that if you use a saw it will regularly need sharpening. A saw is only as good as it's last sharpening. Doing a rip pattern saw is the easiest one to do and for the price of a couple of files (one saw file, one flat file) and a saw set and a £20 tenon saw from ebay you could both have a saw that will cut your long tenons, and develop the technique to maintain your more expensive saws.
Alternatively, like the chisels, North London Saws will do them for very little money (not sure how much as I do my own, but I'd be surprised if it was more than a tenner for sharpen and set).
All that said if I have to do tenons deeper than my saw by hand (which I often do), I split them. I do stop cuts about every inch or so, which is quick and easy because they don't need to be completely square in either direction (just be careful not to saw past your gauge lines), and split them away from the gauge lines, then pare to the gauge line with a chisel near the shoulder and do the rest with a bench plane till I just kiss the gauge lines. It's pretty quick and loads easier to get accurate than trying to saw to a line on a 5" or 6" tenon.
Once again, I am not saying don't buy a Skelton saw, I am glad to see a native craftsman doing well and I'm sure they are a nice thing to own but if what you want is to cut long tenons accurately a sharp £20 ebay saw, a sharp lidl chisel and a sharp £10 Stanley no.4 will do just fine.
Paddy