I have a 1982 Roy Thames, Holdsworth bike to sell

Sorry 26" wheel

That's really unlikely. Have a read of this. It's most likely to be 700c, in which case you'll find something like (XX-622) on the tyre, or 27 x 1 1/4 in which case you'll find (XX-630) on the tyre (where XX is some number between 23 and 32). Let us know which.