I can't believe this thread is still going. I think the main protagonists are both extrapolating from their own experiences and generalising from the particular. Neither of them can have visited every pub in the country.

Taking a broad view, I don't think these signs are common. I travel all over the UK, often visit pubs, and I don't recall ever having seen one. On the other hand, perhaps I'm not visiting the same pubs travellers do, so I wouldn't assume that my own experience reflects theirs, or that these signs categorically do not exist.

When Jim was going around with travellers he would of course be visiting those pubs most likely to have such signs, so perhaps he gained an exaggerated impression of how widespread they are.

My own view is that "No Travellers" is not a "common UK sign", but may be common in certain areas. Of course, one anywhere is too many.