Odd question I think, I can only answer one with certainty and that is the the Shannon is the longest river in ireland, as it is the longest in the British Isles. I am pretty sure the Severn is longer than the Thames so it would qualify as the longest river in Wales and England. But if you mean wholly in the country and not part of an other for England that would be the Thames and some other river in Wales. As for Scotland I am pretty sure that is the Tay and now I shall go and have a look.