Is it too late to plant dahlia tubers?

You MUST plant them now.
Dahlias are plants that are best in the autumn anyway. The main concern might be that the garden centre tubers could be dehydrated by now and might be difficult to get into growth. I'd soak them in warm water first for 24 hours before planting them in a pot to get them going. Plant them out in the garden if you want to once they are well into growth.
The tubers are extremely unlikely to survive until next year if just left. They are living things and designed to come back each year not remain dormant like some seeds.
Even if you don't get magnificent flowers in time (which I'm sure you will) then the plant will have time to make lots of leaf which will give it the 'oomphf!' to make a new tuber for next year. After all it will stay in leaf until November at least, unless there are frosts incredibly early.

I'd be inclined to plant them, but then, I live in West Cornwall and take no end of horticultural liberties!
You haven't told us which county you're in, so assuming you're in cooler climes I think you'd be best keeping them somewhere cool and dry until next year. As soon as the eyes start to appear, then you can plant them.
Also, a warm welcome to GoY.

Welcome Mycroft50.....:¬)

Thank you so much for your advice. I have done as Bertiefox suggested and am now keeping my fingers crossed. Oh, by the way, I live in the sunny West Midlands.