1927 Etching signed by Charles J.Martin

Very interesting image!

There are a few C J Martin's around, including one from the 1800's who has a lot of work still available. One needs to research this further but I think you are going to find that it's C.J.Martin the designer/architect. It was not uncommon for the architect's back then to also do artwork of the projects they worked on.

I see a few lines of attack for this type of image that you can follow up on.
1. Carefully open the back of the frame and look for marks or other information. Sometimes it is recorded and put behind the backing board.
2. C.J. Martin construction Co. exists still, go to their website and see if they can identify it, it might be the founding father of the company! (also, they may even want to buy it from you - given the name and subject matter both match them!)
3. Try and identify the structure. Contact a historical architecture group in Brittan and see if they recognize the structure or the name! I do agree with you that it looks English and a major work, but it may or may not be London Bridge. They did have others and it seems to me that L.B. had more towers and was built earlier.
4. Finally, contact the architect's society of England and America both. See if you can turn up an architect from the 1920's with that name who worked on public projects.

As an unidentified Artwork Print by an unidentified author, given the size and assuming it's original (not a print), you would be looking in the low hundreds of dollars, most likely about $150. If you can identify either the structure or the artist and they are recognizable, you would then (depending on the answer) move to the mid to high hundreds most likely.

If it is identifiably British, your best market would be around where the bridge is built. People tend to be more interested in images of what is around them.

Hope this helps!