Hi Johnny
My gut feeling is that it is probably worth a bit more than £50 ~ have you tried starting it with some fresh petrol. Briggs & Strattons are 4 stroke engines but may need some easy start (https://www.halfords.com/motoring/engine-oils-fluids/fuel-oil-additives/bradex-easy-start-300ml) to get it to fire, but try "turning over" with spark plug disconnected / removed to make sure its not seized. If its running it will be worth considerably more. If you were closer I would come and have a look for you. But back to your original question ~ a google search shows this result.
https://www.google.co.uk/search?q=auctioneers+in+essex+uk&rlz=1C1CHBF_en-GBGB776GB776&oq=auctioneers+in+essex+uk&aqs=chrome..69i57j0l3.34591j0j8&sourceid=chrome&ie=UTF-8Give some a ring and see if its what they would consider suitable for their auction house. If you can get it to them they will be able to give you an estimated value ~ let us now how you get on.