"Royal Winton" was a trademark used by Grimwade's after 1930.
At auction, a Grimwades plate like yours sold recently fr $7.81.
See the following image.
An realistic estimated retail value would be $20-25.
I wish I had better news.
Online auctions like eBay generally bring wholesale selling prices.
If you would like to sell close to estimated retail, some ideas are:
The internet has your largest audience of buyers. Place small ads on these sites or set up your own small store on these sites.
Etsy.com - small fees - 20 cents to list plus 3.5% final price.
Amazon Marketplace: http://services.amazon.com/content/sell-on-amazon.htm/ref=footer_soa?ld=AZFSSOA
A private seller to a dealer or at auction can expect 30-60% of estimated retail value.
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