Tthe bike is a Sunbeam S7 or S8. One of those models used the large tires as seen, the other used a smaller front tire, I can't remember which.
It was an overhead cam in line twin, noted for smoothness and not much else. The ammeter was on the right side cover, under the seat. It required one to stop, get off and read the thing. Only the English!
These bikes were not noted for reliability if run hard, i.e, anything over 50 MPH was hard on the drive train. The rear cylinder often overheated. The drive shaft used a non-locking worm drive that wasn't always non-locking.