Barr & Stroud were one of the many providers of mid range optics during WW2 and are still going today.
A good pair with a good leather case can fetch up to £120-150 with some provenance, but as there are so many that are swamping the market currently, especially on that site, most have been selling for under £50. I would take a look around these sites so you can best gauge what you want to pay for them IMHO.
If it was a friend of mine and I was interested in collecting them, I would probably offer £100 which I think would be fair and honest!