Hmn, difficult one with the heartwormers, I would be tempted to bring it- I am pretty sure veterinary supplies are subject to various restrictions.
I will try to find out from a colleague with endurance horses, but I think it would be a lot harder to get the kind of stuff here that you can get in your farm shop.
Another option here, for regular items, is to ask your vet for a private prescription (you may be charged for it, but it's minimal, last I knew it was about £3-5) and you can then go to an online pharmacy and save money that way. *most* vets will not let you know you can do this, but if asked, there are new guidelines out that they shouldn't reasonably refuse
http://www.vet-medic.com/hierarchy.aspx?strGrp1Code=HORSSo, you are bringing your horses over from Oregon to the UK? (it's weird, one of my far off dreams involves having horses again one day and living in Oregon ~ not that I have been, but it seems to be the kind of place I would be happy!