Premier equine do a fab full neck stable rug - you'd probably have to phone them for a heavier weight as they only have lighter stuff on their website just now. Alternatively, if you're feeling very rich, I love the FAL full neck rug.....
I have a Mark Todd stable rug that you can put a full neck on. Expensive but well worth the investment I've had it 4 years and it hasn't got thinner or ripped like most of them have.
I am really surprised that so many people go for full neck rugs. The only one of my rugs that has a neck, is the Boett that she wears during mozzie season. In the winter my horse, who is a TB and has a very high trace clip, would be hot and uncomfortable with a full neck rug.
I live in north scotland, by the coast with all year permawind! My horses freeze when they're clipped if they don't have big rugs on! That said, the youngsters all have standard necks....
i have a mark todd under rug with neck that is 250g.
it has full fastenings so can be used as a 'proper' rug but its just much cheaper as itsnt fancily finished.
i them used an amigo LW stable rug on top to make a 400g heavyweight
i found this worked really well, and was a more cost effective way, as now i have LW, MW and HW options, for all year round, with just two rugs
the amigo is fab, washes really nicely in the machine
John Whitaker 'Lactic' stable rug is far far away the best rug I have ever bought for my large IDxTB. It was about £70-80 and is massively better made and better fit than anything else, even has adjustable neck. If your horse has big shoulders, its great and never rubs and is VV smart and washes up like new. You can prob get one like mine cheap too as they have since updated the design but I prefer the old design - oh and the outer shell is waterproof and ripstop too so rug trashers are defeated!