Down Home Music in El Cerrito is a great idea if you are in the Northern California area. I second that one. But as far as a price list goes, it's almost impossible for there to be a current listing; so much depends on who wants what and how badly. To take the Transports example you cited, since it has been reissued on CD and is now more generally available, the value of the original 2-LP set will depend on how much someone wants it. And this is going to be true of most of your folk LPs; if they are albums which NOT out on CD, then they will be of greater value to those among us who love this stuff; but if they ARE out on CD, fewer people will want them other than as artifacts - and btw fewer people these days even have a turntable to play them on.
Of course if you sell them to a shop such as Down Home they will give you less per item since they have to mark them up to sell; why don't you create your OWN listing of what you want to sell and offer to email it to anyone who might be interested? The offers you get might give you a good idea of what things are "worth" to this group, anyway, and if the offers seem too low, you are under no obligation to sell.
I would think that shipping 600 LPs from the US to the UK would be prohibitively expensive because of the weight.