There are a lot of resources online for British hallmarks, especially Birmingham. This website also has a page dedicated to Researching British Hallmarks. 18K is a nice high carat, but a plain ring such as this is probably not extremely valuable unless you know something special about its provenance, such as it belonged to Churchill's father. $100-300 depending on the weight I would guess unless the maker is highly sought-after.
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Antique Wedding Ring by: Anonymous
Hi Allen
the WM letters on your gold wedding band, are the maker's mark and as you mention, there are many makers with these two intials. But the frames are different, the font style, etc, also vary and some have a decimal point in between the letters. If you can determine the date year and more information about the maker's mark, this information will help to nail down the jewelry manufacturer.
Allen, FYI high quality gold, 18ct gold and higher was the traditional choice for UK gold wedding bands of the past. The Crown and the number 18 is a UK purity mark and has been in use since 1798, earlier than the Crown and 22 which has been used since 1844.