Jean, your grandchildren will be worth a fortune when they go to the Antiques Roadshow of the future.
Only if they've got the complete set. 
They have a 30th coin but with the price they are charging I think I will stick to the 29
You've already spent (or saved, or invested, however you like to look at it) £14.50. The extra medalion, to complete the collection, is only another £3. It must be well worth that. If no-one bothers to buy the completion medalion, then that's the one that will be rare and valuable.
It is a bit disheartening to see some of these antiques shows where grandchildren have come into possession of family heirlooms, and the children are not the least bit interested in keeping them, to pass on themselves. They just want to sell them. Presumably, many years later, some of them will realise their big mistake.