Hi Joanne
You don't say whereabouts you are. A fair place to sell them would be at a rare breeds sale, where they usually have a section for unregistered and cross-bred stock at the end of the sale. If you are in Scotland, the Stirling Rare Breeds Sale with The Hebridean Sheep Society Scottish Show and Sale is held at Caledonian Marts towards the end of September (last year 23rd, previous 25th)
Again, if you are in Scotland, there is a fair trade in unregistered Hebrideans, especially the further north and west you go. I don't know what the situation is in England and Wales, but judging by messages on TAS there are plenty of people happy with unreg stock down there.
It would seem to be a perfect opportunity to rear the lambs to eating size (best at 16 months for Hebs) and sell on the adults.
You might also do well selling by advertising in the farming papers, or even in your local feed store.