It is a "telescope, sighting, no33". You are right, it would be for an armoured vehicle like a tank that has a 2 pounder or 3" gun, and a coax MG. Here is a pretty good link to a listing off all these telescope, sighting:
http://www.wwiiequipment.com/index....ting-telescopes&catid=49:other-data&Itemid=61
As to value, there are thousands more of these telescopes around than there are relevant armoured vehicles. If you check completed auctions for "telescope sighting" on ebay, you will see this pattern of scope range from $30 to at best a couple hundred. Yours is missing the leather eye-piece. Since it is for a vehicle and not a gun, I would suggest the value is towards the lower end of that.
The same basic telescope but with different reticles could be found on the various artillery pieces as well. I have a no33 that was modified over to no42 for one of my pieces.
REL was a Cdn Crown corporation that made 10s of thousands of optics, both for us and for the commonwealth. They were also involved with early radar design and production.